October 29, 2010

Ellen Carrlee, Conservator, Alaska State Museum

2003 move of the Auk Tribe Pole from Centennial Hall in Juneau, Alaska. Now located in the Juneau Douglas High School.

In the period of 2000-2010, I have seen four totem poles moved in Juneau, Alaska.  Because totem poles continue to be made and placed in outdoor environments, deterioration will eventually occur and totem poles will need to be taken down for treatment or relocation.  This posting is written to help folks charged with the care of historic totem poles to prepare for the slightly nerve-wracking experience of taking down a totem pole.  Once you’ve identified some professional expertise (a structural engineer and a rigging specialist are good folks to have on your team) here are some considerations.


In the case of a 4,500 pound chunk of wood hanging from a wire, this is especially crucial.  There ought to be one person who is calling the shots, and one of the most important shots is, “SLOW DOWN!”  Taking down a totem pole takes around three to five hours if all goes smoothly.  There is a tendency in the excitement of the moment to rush.  Checkpoints for human safety ought to include:

Four Story Totem pole coming down. Photo courtesy of the Juneau-Douglas City Museum


Condition of the pole should be documented beforehand as well as possible, using digital images and binoculars.  Try to determine any areas of rot or parts like wings and beaks that may be separate pieces.  Those joins are sometimes weakened over time.  Note the locations of areas of carving, paint, and particularly protrusions that will need to be protected.

Height measurement is needed to determine things like how the pole might fit on a 40’ flatbed truck, what building it might fit into, if corners can be turned, and so on.  An inclinometer is a device to take that measurement.  Occasionally there is historical documentation from when a pole was carved and erected that describes its full height.  Newpapers, for example, are one source of that information.

Weight estimation is useful to determine how much stress will be placed on trucks, wheels, floors, lumber, etc. Western Red Cedar, a common wood used for totem poles, is approximately 23lbs per cubic foot with specific gravity of around 0.32 and 12% moisture content.  Wetness of the pole adds some weight and can be measured with a moisture meter.  The calculation is most securely done by an engineer.  For the YaxTé Pole in the Tongass Rainforest near Juneau, US Forest Service engineer Scott Jackson estimated the pole to be 47 ½ feet high using an inclinometer and measured a moisture content of 18-20% near the surface and around 35% in its center.  The estimated the weight of the pole (with its support beam for lifting) to be close to 5,000 lbs.  During the move, the crane computer indicated the pole weighed around 4,500lbs (+/- 200lbs)  The stump had a moisture content around 32%.  The measurements when the pole was down indicated the pole was actually about 48 feet high with a base diameter of 2’9”.

Measuring moisture not only helps you determine the weight of the pole, but also how rotten it might be.  There are two main kinds of probes, a pin probe for measuring the surface condition and the hammer probe for measuring deeper.

Pin probe for measuring surface moisture content

Hammer probe measures moisture content deeper in the wood


Bucket lift. This is helpful in performing inspection of the upper areas of the pole, where the lifting is most likely to happen.  The lift is also to put on the padding material and placing the straps for lifting

Bucket lift with plywood in place to protect the grass

Backhoe Could be helpful to dig out around the base of a pole that has been erected directly in the ground.  Not strictly necessary in all cases

I didn't know what exactly a backhoe was until I had a two-year-old son. Now I can recognize one half a mile away.

Boom truck or crane. A boom truck typically has a long arm with one pulley/hook at the end for lifting.  A crane is usually much bigger and can lift in several places if necessary.

The boom truck or crane will work together with the manlift.

Flatbed Truck. Typical size is 40 feet.  May be called a “40 foot flat” or sometimes a “lowboy” if the wheels are on the ends and the flatbed is closer to the ground.  A lowboy is often used to move around backhoes and other heavy equipment to construction sites.  Flatbed trucks are also used to move around large shipping containers, for example.

This flatbed truck from Alaska Marine Lines is usually used to move shipping containers that come to Juneau on the barge.

Strongback This is usually a wooden beam or box beam that goes up most of the height of the pole to act like a splint.  A box beam is a beam that has four sides like a box and is hollow in the middle.  A beam might be a long 4×4 of solid wood, or it might be a glu-lam beam (made of many pieces of wood laminated together with glue in a factory.)

This beam up the hollowed back is lag bolted to cross braces, then spacers of wood were put in the gap, and the whole thing held with straps around the pole.

Straps. Wide nylon straps (at least 3-4” wide so it doesn’t bite into the wood) are used to lift the pole.  Sometimes they are fashioned into a loop and lift the pole that way (sometimes called a choke or choker.)  Sometimes a slip buckle is used.  A slip buckle has the advantage of releasing tension when it is not weighted, but it is usually metal and care must be taken to protect the surface of the pole from damage.  The strap will be hung from the hook of the boom or crane, but there is usually a heavy metal ball above the hook, so sometimes an extension is put on the strap to keep the ball away from the pole.

Example of a strap used as a "choker" to lift the pole. Photo courtesy of the Juneau-Douglas City Museum.


Instead of a loop around the pole, these straps have slip buckles

Extra extension on the strap keeps the ball away from the pole

Cribbing It is a good idea to have plenty of wood scraps on hand, 2×2, 4×4, 2×4, plywood etc that can be used to support or “crib up” the pole when it is coming down.  In the case of the YaxTé Pole, the USFS had an entire pickup truck full of wood, and it was very useful.

Having a variety of scrap wood on hand allows you to custom fit your cribbing under the pole as it is lowered slowly onto the flatbed.

Padding This is one area where the artwork issue really comes into play.  Packing blankets, soft foam, and carpet squares all make good padding and should be used wherever rigging and strapping materials could damage the pole.

Ropes are needed to help secure padding to the pole as well as belay ropes tied around the base

PPE (Personal Protective Equipment) Wearing hard hats and orange vests help maintain visibility, remind people to focus on safety, and to make people who may walk into harm’s way stick out because they are not wearing protective gear.  Gloves prevent splinters, rope burn and can improve grip.  For chainsaw operators, the gear will be more extensive, such as chaps.

Also handy: radios or walkie-talkies, binoculars, cameras, tape measures, traffic cones, Do Not Enter tape, duct tape (who knows?  Always have duct tape in your uh-oh bag).


Again, remember it is crucially important for someone to be in charge, indicate when things need to slow down, supervise safety and make sure the effort is happening in a controlled manner.  The person in charge ought to be assigned that task by the legal owner of the pole, since this is the person who will probably deal with the fallout if things go wrong.

A good crane or boom truck operator has great skill and regularly performs delicate maneuvers.  Obviously, one should not offend the operator by telling them how to do their job.  However, most have never dealt with moving artwork, which has specific requirements and fragility.  The museum professional and the rigging professional come from different worlds, and ironing out the plan ahead of time will be invaluable.  The day of the move, there is adrenaline and momentum that takes over and makes it difficult to slow things down or change course in a thoughtful, deliberate way if things are not going smoothly.  Prepare for this.


The strongback is the most essential element to moving a pole.  A wooden or metal beam or box beam acts like a splint and augments the strength of the pole to help it move safely.  The moving process tries to put the stresses on the strongback instead of the pole as much as possible.  The beam is held close against the back of a solid pole, or secured in the cavity of a pole that is hollowed out in the back.  Additional cross bracing and spacers may be needed to help the beam stay in place inside a hollow and deal with compression forces as the pole is moved.  The entire strongback structure may be strapped to the pole with nylon straps and tightened with ratchets.  It is a good idea to put padding between the nylon straps and the wood surface, and especially behind any metal elements.  Of course, if the pole already has a strongback of some kind, that can be utilized in the support structure for moving the pole.  A strongback can easily add several hundred pounds to the overall weight.

Padding is one of the aspects that is often overlooked.  Carving, painting, deteriorated wood and weathered surfaces are all vulnerable to damage by gouging and abrasion (or worse) during the move of a totem pole.  Anywhere metal comes in contact with the pole ought to be padded, and places where straps may put compression forces on the pole are also good locations for padding.  Consider where the crane’s overhaul ball may be.  That is the stabilizing ball just above the hook that keeps tension on the line when the hook is not loaded.  You don’t want that smacking into your pole.

If you cannot arrange to have the ball clear the pole, be sure to protect the pole with padding.

There are several ways to lift a pole and put it horizontally on the ground or lay it on a flatbed truck.  There is usually one “pick point” near the top of the pole where a strap is placed on the pole to lift it up.  Sometimes that is called the “choke” or the “choker” because it squeezes around the pole and makes compression forces.  Padding the pole at that point and anticipating where the ball might be is key.  Often the tops of poles have beaks, wings, or other protruding elements that must be protected from damage, especially during moments when the pole swings free.

Several belay ropes ought to be placed at the bottom of the pole with people manning them from a safe distance.  This will allow for control of the base of the pole when it is freed from the ground, and prevent the pole from swinging or spinning.


No need to rush here.  Human safety first, protecting the pole next.  Once the pick point is set on the top of the pole and belay ropes are ready, the pole can be freed from the ground.  Sometimes a pole is not buried very deep, so it can be dangerous to dig up the support around the base of a pole without having another way to prevent the pole from toppling over.  If the pole is still in the ground, it is a good idea to dig down far enough and wide enough to allow a chainsaw to cut off the pole at least 6” below ground.  A backhoe can help with some of this chore.  More can be trimmed off later, if necessary.  There is often an area of rot right around ground level, down 12-18” or so, and then further down the wood tends to be sound again.  Trying to pull all the “root” of the pole out puts stress on the pole and is not necessary, since the recommendation is to mount reinstalled poles with a strongback and a gap between the support under the bottom of the pole and the ground to prevent future rot at the base.  Sometimes, as in the case of the Four Story Totem, a crane allows for a second pick point to be placed near the bottom of the pole, so it can be lifted into a horizontal position right away.

Crane with two pick points on the Four Story Totem pole. Photo courtesy of the Juneau-Douglas City Museum.

In the case of the Auk Tribe Pole, only one pick was used at the top, and then the base of the pole was set on the ground and the top slowly lowered to cribbing on the ground.  After it was on the ground, a second strap was placed near the bottom and the boom truck lifted the pole in its horizontal orientation and placed it on a flatbed truck.

Slowly lowering the Auk Tribe pole, with the base on the ground and cribbing ready to support the pole.

Two picks used unload the Auk Tribe pole from the flatbed and set it on a rolling dolly.

The YaxTé Pole method was to have a pick point from the top using a boom truck, and then use the snatch hook on the bucket of a backhoe to lift the strap on the lower part of the pole.  On this pole, the lowest 25% or so of the pole was riddled with carpenter ant damage, and it was not certain there was enough structural stability to pivot from the base and be laid down on the ground.  In most cases, the cranes, boom trucks, and other lifting mechanisms are used to hold the pole in the air in a horizontal position until the flat bed truck can be driven underneath and then the pole is gently lowered onto the flatbed.  Supporting the pole underneath in as many positions as possible is very important, and this is where a large supply of cribbing material comes in handy.

Snatch hook on the bucket of a backhoe used to lift the end of the YaxTe pole. A little longer sling would have given a more comfortable clearance between the teeth and the pole.

The boom truck lifts the top of the pole and a sling on the backhoe lifts the base while the flatbed is backed underneath


If the pole is simply to be laid on the ground and worked on nearby, adequate cribbing below, covering from the weather, and protection from vandalism are the main aspects to consider.  A pole will be the most stable flat on its back, so cribbing at a height that workers can access the back of the pole from beneath is an important consideration.  If the pole needs to come indoors for work, there are a few more steps.  One is to build a strong dolly to support the pole on wheels in order to roll it indoors.  Heavy duty dumpster wheels are very strong and rated for big loads.  Having a dolly the full length of the pole provides another kind of strongback for the pole, supporting it fully and maximizing maneuverability.  Smaller individual dollies may also work, but can be more difficult to control and don’t provide as much support.

Dumpster wheels attached to the underside of a full length dolly.

Smaller individual dollies can also be used, but if the ground is uneven and some of the wheels come off the ground it is a little harder to maneuver.


In the Tongass Rainforest of Southeast Alaska, totem poles are often quite wet and need a few weeks indoors to slowly dry out and come to equilibrium with the indoor temperature and relative humidity.  Cover the pole loosely with plastic sheeting to make sure that process does not happen too quickly, and if possible, turn down the temperature indoors to the 50-60F range.  If the drying process goes too fast, the pole is at risk of additional cracking and damage.

Auk Tribe pole on dolly inside the atrium of the Juneau Douglas High School

Plastic sheeting over the Auk Tribe pole to slow drying. Barrier around pole to prevent touching or vandalism.

My sincere thanks to several people who have helped me to understand this process.  Bob Banghart is the Head Curator at the Alaska State Museum and was in the private sector as Banghart and Associates for many years previous.  He has moved several totem poles since I’ve been in Juneau and  is the first person I go to with rigging questions.  It was my realization that he won’t be around forever and someday I might be asked how to take down a pole that led me to write this posting.  I’m also grateful to Ron Sheetz for his wisdom and generosity regarding totem pole conservation.  He has taken many poles up and down throughout Southeast Alaska.  He is a “retired” furniture and wooden objects conservator who worked for the National Parks Service for many years.  He still comes to Alaska to take care of totem poles and try to pick his brain the best I can at every opportunity.  Al Levitan and Andrew Todd are two other conservators with lots of totem pole experience, although I have not been lucky enough to pepper them with questions as I have Bob and Ron.  Addison Field, curator at the Juneau-Douglas City Museum, has also been very helpful in sharing his experiences in caring for the Four Story Totem pole at his museum.


October 21, 2010

PEM2 datalogger and HOBO Pro Series logger

A Review of Recent Ideas and Annotated Bibliography

Ellen Carrlee, Alaska State Museum Conservator, October 2010


For a long time, the climate ideal for museums was 50%RH and 70F with very little fluctuation.  This has been problematic because it costs money and fossil fuels to achieve that target, and many buildings simply cannot achieve it.  In addition, there have not been enough scientific studies on real materials to prove that this is the correct target.  The new way forward in thinking about museum climate involves an understanding that deterioration happens by different mechanisms (biological, chemical, mechanical) and that you have to balance the risks and benefits for each on.  Very low RH slows down biological and chemical risks, but increases risk of mechanical damage, for example.  The beginnings of a shift in our conception of appropriate collections preservation climate began in the mid 1990’s, but it has taken longer to reach the dawning of a new era.  A review of the recent literature suggests that a summary of the subject can be split into three areas: causes of damage, methods of analysis, and recommendations by material.

In thinking of the needs of a future integrated facility for the Alaska State Library, Archive, and Museum, I am struck by how the library/archive needs are really quite different from the museum needs.  To oversimplify, I think it is because the threats to paper are mainly biological and chemical, and therefore they are less concerned with RH and more concerned with temperature.  In general, they need RH to be lower than we like it at the museum, and fluctuations are not as threatening.  Our museum, with lots of paintings and organic ethnographic materials, is keen to have a bit more RH.  There are competing needs that have to be weighed.  Light fading risk goes down at low RH, but fracture risk goes up.  Higher temps reduce RH and therefore help prevent mold but low temps reduce chemical self – destruction.  Just as we’ve looked at light fading risk and recommended light levels for different categories of materials, we’ve started to consider temp and RH needs based on materials.  But that’s another posting (and a lot more reading for me!)


  1. Biological
  2. Chemical
  3. Mechanical/ Physical

Some people will say that RH is more important than temperature, and others will say just the opposite.  Seems like the answer depends on what kind of damage you want to prevent.  If it is biological, you need to keep your RH low.  There is an easy-to-remember rule of thumb that mold tends to flourish if you have stagnant air in a dark place over 70% RH and 70F for at least two days.  Also, insects and pests need moisture to live.  So for the biological risk, looks like temperature has the edge.  Chemical usually refers to rate of deterioration, like things becoming more acidic or corrosion happening.  It is hard to say if temp or RH is more important here.  Some sources will say that for approximately every 5C DROP in temperature, you double the life of your artifact.  Anyone remember the Arrhenius Equation from chemistry class?  That one says for every 10degrees C the rate of a chemical reaction doubles. Think about how fast sugar dissolves in hot tea and how slowly it dissolves in iced tea.  So temp is definitely important for those chemical reactions that cause degradation, but a lot of those equations involve water.  So RH is important, too…metal corrosion, dye fading, lots of things.  For chemical changes, short fluctuations in temp/RH don’t matter much, it is the situation over the long haul that is most important.  In general, room temperature is too warm for ideal long-term preservation of most organic materials as well as paper, leather, dyes, and film.  Mechanical damage seems to really connect strongly to RH and damage from movement caused by shrinking and swelling as moisture content goes up and down.  Now some of that risk is influenced by how brittle something is, and things get more brittle as they get colder, so temperature has a role, too, but not quite as much as RH.  Some sources separate out “physical” as another category to show how something might get torn apart because there are various materials all restrained in association with each other (composite or mixed-media), like drums or paintings, but this seems to fit OK in the “Mechanical” category.  James Reilly has a good description: To avoid mechanical damage, stay in the 20-70% RH range and keep outside excursions as short and infrequent as possible.  Seasonal extremes are worse than short-term events.  Short term excursions don’t matter as much because materials take quite a while to equilibrate, weeks and not hours or days.  The Smithsonian folks are more conservative with RH and in general suggest 30-60% as being OK for most collections.


How do they know what is really causing damage?  How do we know that damage didn’t just happen before it even came into the collection, or as a result of inherent vice in the material itself?  Apparently, those original targets of 50% and 70 degrees F were based on trying to replicate conditions of a slate quarry in the UK where valuable collections of the British Museum were kept during WWII, and the allowable fluctuations were based on the limitations of the HVAC equipment at the time.  When I was in school (late 1990’s) I was taught that what we really wanted was to keep that line on the hygrothermograph as flat as possible, and that big, rapid fluctuations were really bad for the artifact.

Proofed RH fluctuations Michalski the idea in a nutshell is that you look at the highest and lowest extremes of RH the object has been exposed to in the past, and then you avoid ever getting to that boundary again.  Whatever damage happened at that limit has happened, and as long as you stay away from that extreme, you’ll probably be OK.  Let’s say there was a crack in a wooden item that happened at low RH.  If you keep away from that extreme, the crack will probably just open and close with RH fluctations without getting any worse.  If you get closer to the extreme, the number of times that crack can open and close without consequence is limited.  Remember, this is just for mechanical damage.  That’s not the case for biological or chemical damage at all.  Also, conservation treatments may erase that proofed RH because they introduce new materials  and stress distribution in the object.  I have especially seen this in the treatment of large organic materials like kayaks…they show no major change for years before conservation treatment, and then in the year or two following treatment you see changes.  If the treatment was done right, those will result in the repairs opening up or flexing and not in new damage to the artifact.  I’ve come to think of these artifacts as experiencing a “period of convalescence” after treatment.

Isoperms Don Sebera’s work in the early 1990’s, looking into relationships between temp, RH and preservation as a matter of activation energies for decay reactions.  Quantitative graphical measures of relative permanence, such as if paper will last 100 years at the accepted standard, it will last only 3 years at 95F and 80%RH, but 1200 years at 50F and 40%RH.

IPI decay metrics  the Image Permanence Institute (a non-profit research lab at the Rochester Institute of technology) has done amazing work (grant funded by the NEH, the Mellon Foundation and IMLS.) to quantify things.  Great quote from James Reilly: “One cannot manage what cannot be measured”  An important aspect of this new way forward is to realize it is based on algorithms, modeling, predictions and other scientific ways of guessing how materials are supposed to behave.  Since deterioration mechanisms of each material are complex and dynamic, it is difficult to get real info about damage to real artifacts.  The whole IPI scheme is a great hypothesis, and maybe the best one we’ve got, but there is precious little in hard data and proof.  For each of the three forms of RH/ temp deterioration, they’ve picked something to be the canary in the coal mine and then projected possible damage to vulnerable organic materials using math.  Biological decay is predicted by mold growth, chemical decay is predicted by a model of deterioration for organic materials based on the hydrolysis of cellulose acetate (common plastic used for photographic film), and the physical decay is predicted by the equilibrium moisture content for an imagined block of wood from an “average species.”  Tim Padfield has a good article describing the science behind the preservation index and the TWPI

The Marvelous Marion Mecklenberg  OK, I have to be uber-caffeinated to read anything he writes, but the man has done great work, co-authored with many really stellar people and in general, when I think of cold hard data I think of Dr Mecklenberg and his colleagues,  in particular Tumosa, Erhardt, and McCormick-Goodhart.  They did research starting in the early 1980’s to measure dimensional changes to different materials with mechanical testing to predict stresses and strains, as well as modeling to show how much climate change was required to cause irreversible changes.  In 1994, they announced new guidelines for museum climate, suggesting that 50 +15% moderate fluctuations were OK, and that for general collections, 30-60% RH was mechanically safe.  Temperature in the human comfort range was fine, but avoid going below 13C (55F) due to some materials undergoing phase change and getting brittle, like acrylics.  Of course, some materials need more stable environments, like degraded materials, drums, veneers etc.  The current Smithsonian guidelines as of publication were 45 +/-8% RH and 70 +/-4F. 


My understanding of the current trend is based on the articles below.  The links were good as of October 2010, but if they fail for you, just try googling them.

____________“Rethinking the Museum Climate”  (2010) Meeting, April 12-13, 2010 MFA Boston

____________ “The Plus/Minus Dilemma: A Way Forward in Environmental Guideleines”  (2010)  3rd IIC Roundtable, 13 May 2010, Milwaukee Wisconsin at the AIC annual meeting

In addition to the balancing ideal preservation environment with the comfort needs of visitors and employees in the same spaces, there is growing influence of economic and environmental concerns (carbon footprint) since trying to maintain a rigid target is resource-intensive.  Loan requirements are more strict than many institutions can achieve, and institutions often ask that the borrower keep more rigid standards than the lending institution can manage.  We need to be more honest about what are actually able to pull off.  Michalski thinks a 10% fluctuation is OK  He thinks the biggest issue is mixed media or composite artifacts.  National Gallery of Canada is now going with 44% RH +/- 3% in winter and 50% +/- 3% in summer, with a year-round goal of 71F +/- 2%

American Society of Heating, Refrigeration and Air-Conditioning Engineers. (2007) Chapter 21: Museums, Galleries, Archives and Libraries. ASHRAE Handbook, Atlanta, GA

The 1999 edition included a section on Museums Libraries and Archives for the first time.  Revised edition in 2003.  Michalski was lead author for the T and RH section.  Next one due out in 2011 will probably include more about “proofed fluctuations.”  The introduction here describes threats in decreasing order of seriousness: light, RH, temperature, air pollution, pest infestation, shock/vibration, natural disaster, theft/vandalism and misplacing objects.  IPI preservation index is specifically mentioned.  Lots of detail and references cited in the sections regarding biological, mechanical and chemical damage.  Conservative RH for no mold to occur at any temp on collections is under 60%.  Mechanical damage section seems to point toward +/- 10% as being a maximum acceptable RH fluctuation, with damage being reported when the fluctuation was closer to 20%, especially on cracked cabinetry and paintings.  Chemical damage section focuses on acid hydrolysis of paper, photos and magnetic media and recommends cold and dry but without a specific range given.  Response times of artifacts suggest that anything under an hour is probably not an issue, such as a 15 minute HVAC cycle, as long as dampness is not promoted.  In the design parameters section, Class A is considered optimum for most museums, with AA being high in energy costs without a lot of benefit.  10% seasonal swing in RH and 5% short-term fluctuation is OK.  Temperature set between 59 and 77F. Constant air volume is recommended.  Variable air volume tends to have poor humidity control, inadequate airflow, maintenance disruption, leaks, and inflexibility and is not recommended for collections housing.  The next version of this handbook is expected to come out in 2011.  Most engineers and architects are familiar with temp and RH issues only though this handbook, and conservators are wise to be familiar with its contents.

Anderson, Maxwell. (2010) Revising the Gold Standard

Brief article for general audiences, written by the director and chief executive of the Indianapolis Museum of Art.  There are three considerations of RH: set point, allowable fluctuation, and seasonal adjustment.  Some discussion of where the old target standards came from.  IMA 50% +/-8% (6% per 24 hours) and 70F +/- 4 (2 per 24 hours) 

ANSI/NISO Z39.79-2001 Environmental Conditions for Exhibition of Library and Archive Materials.  American National Standard developed by the National Information Standards Organization.

Set point for temperature should be below 72F with a variation of up to 5degrees on either side of your set point.  RH set point should be between 30 and 50% with a variation of up to 5% on either side of the set point.  Seasonal drift is not to exceed 5% per month. 

Bizot Group draft Guiding Principles (Int’l Group of Organizers of Large-Scale Exhibitions)

This organization is made up mostly of directors of large museums, and these are the people who will ultimately decide what temp and RH limits are in loan contracts between museums, and therefore the key to the big game of chicken: who will change their standards first?  Will other institutions still lend to that daring pioneer?  The IMA has already taken a step toward less restrictive temp and RH limits.

Chicora Foundation (1994) “Managing the Museum Environment.”

For each 14 F rise in temp, double the rate of deterioration for paper.  Humidity is more important than temp and should be controlled first.  In contrast to some other sources (Reilly 2008) this article advocates making the humidity the controlled factor and allow the temperature to fluctuate.  Aim for 40%-55% with a daily fluctuation of +/- 3% and 65-75F with daily fluctuation of +/- 5%.  These are not necessarily the ranges we are still talking about in 2010, but specifying how much fluctuation is allowed per day is helpful.  A timeframe needs to be included when you are talking about fluctuation.  Discussion of isoperms.  Really interesting section on the major HVAC components.  A Constant Air Volume System properly filters air and keeps flow to prevent stagnant pockets of air, as opposed to a Variable Air Volume system, which delivers heated or cooled air to each zone and is cheaper but creates humidity instability.  Hmmm, how much humidity instability I wonder?  Electronic controls are more responsive than pneumatic, and the controls ought to be in the occupied spaces and not within the ductwork itself.  I need to compare this to what ASHRAE 2007 says….

Conrad, Ernest A. (2007) “Climate Control Systems Design and Climate Change.”  Contribution to the Experts Roundtable on Sustainable Climate Management Strategies, held in April 2007, in Tenerife Spain.

Author has a master’s in environmental engineering and has been designing systems for major museums and libraries for 25+ years.  Apparently, mechanical engineers look at statistical weather data, mostly published by ASHRAE in 30-year averages since 1950.  Humidistatic heating often used in historic buildings where they don’t want to add in moisture…instead, they just drop the temperature to put RH in the appropriate realm.  Mentions the New Orleans Charter for Joint Preservation of Historic Structures and Artifacts (1990-91) which was apparently written to establish a philosophy regarding balance between protecting collections, historic building structures and comfort of occupants.  Thrust of the article was to discuss impact of a 1 degree C global increase in temperature.  Mostly it seems that the risk of high RH could increase from an expansion of the tropical zone, and more dehumidification would be needed, thus causing more pollution. Greater risks are associated with increased incidence of extreme weather.  While the author seems very expert in his own field, the topic of the article seems to be more about speculative effects of climate change, which is not his realm of professional expertise, and he doesn’t cite references.  

Conrad, E. A. (1996) “Environmental Monitoring as a Diagnostic Tool.” In Preservation of Collections: Assessment, Evaluation, and Mitigation Strategies. Papers presented at the workshop, Norfolk, Virginia, June 10-11, 1996. Washington, D.C.: the American Institute for Conservation of Historic and Artistic Works. pp. 15-20.

Conrad, Ernest A. (1995) Balancing Environmental Needs of the Building, the Collection, and the User. East Norwalk, CT: Landmark Facilities Group.

Erhardt, D., C. S. Tumosa and M. F. Mecklenburg, (2007) “Applying Science to the Question of Museum Climate.” In Museum Microclimates, Contributions to the Copenhagen Conference, 19 – 23 November 2007, ed. T. Padfield and K. Borchersen. National Museum of Denmark, 2007: 11-18.

If you need to cut to the chase, read this article and then the ASHRAE chapter.  Lays out the history of the traditional standards, which had little scientific evidence and were best guesses and outside factors like limits of HVAC systems, buildings, or local climates.  The Boston MFA installed heating, air washing and humidification in 1908 and determined that 55-60% was the best RH for paintings.  Air conditioning technology improved, and by 1941 it was seen in the National Archives and Library of Congress, among other institutions. The WWII slate quarries in Wales so often mentioned used heating to achieve an RH of 58%, the average RH in the National Gallery of London from measuring the equilibrium moisture content of blocks of wood in the gallery. Some places like Canada used lower RH values (around 25%) because higher values were hard to maintain in winter. The idea that objects were “use to it” was used as justification without scientific proof.  There is also a prevalent notion that more constant is better, although the +/- 2% fluctuation sometimes dictated is hard to even measure accurately with the methods we commonly use.  “Specifying climate control requirements and telling the engineers to implement them is easier by orders of magnitude than the research required to justify the specifications.”  Mecklenberg and his colleagues, in particular Tumosa, Erhardt, and McCormick-Goodhart) did research starting in the early 1980’s to measure dimensional changes to different materials with mechanical testing to predict stresses and strains, as well as modeling to show how much climate change was required to cause irreversible changes.  In 1994, they announced new guidelines for museum climate, suggesting that 50 +15% moderate fluctuations were OK, and that for general collections, 30-60% RH was mechanically safe.  Temperature  in the human comfort range was fine, but avoid going below 13C (55F) due to some materials undergoing phase change and getting brittle, like acrylics.  Of course, some materials need more stable environments, like degraded materials, drums, veneers etc.  The current Smithsonian guidelines as of publication were 45 +/-8% RH and 70 +/-4F. 

Grattan, D. and S. Michalski, (2010) “Environmental Guidelines for Museums – Temperature and Relative Humidity (RH).” Canadian Conservation Institute. October 2010.

Current trend away from a target and toward embracing a range.  Linking RH fluctuation to measurable damage.  Cooling makes RH go up, warming makes RH go down.  Damage is three kinds: biological, chemical, and mechanical.  Fluctuation is the main threat, and your “class of control” defines the allowable degree of fluctuation.  Libraries and archives ought to keep things cool as long as it is not too damp.  Around 30% RH or less.

Hughes, Susan.  UK National Archives, Conservation DistList posting July 15, 2010

The British Standard Institution (BSI) is working on new standards packaged as Publicly Available Specification (PAS 198) for environmental conditions in museums libraries and archives, which will influence storage, display, and loans in the UK.  Expected out in May 2011.

Image Permanence Institute (2005) Step-by-Step Workbook: Achieving a Preservation Environment for Collections.  Image Permanence Institute, Rochester Institute of Technology.  Rochester, NY August 2005

As opposed to the “target” set point recommended for many years, the new way forward in temp/RH involves an understanding that deterioration happens by different mechanisms (biological, chemical, mechanical) and that you have to balance the risks and benefits for each one…very low RH slows down biological and chemical risks, but increases risk of mechanical damage, for example.

They have a holistic approach to the preservation environment, which includes a Preservation Management Process: 1. Understand 2. Evaluate 3. Take Action.  As part of understanding, it includes recommendations for how to understand your own HVAC system.  The ”schedules” section of mechanical plans are the part you want.

Great explanation of the importance of dew point.  The temp at which water will condense out of the air, so it is a measure of the absolute amount of water in the air.  If you have that amount of water outside when it is cold, the RH will be pretty high, but then bring it in and warm up the air and the RH goes down.  An example I looked up for rainy Juneau Alaska: if it is 40F and 90%RH outside, that’s a dewpoint (absolute amount of water) of 37.  Take that inside and heat it up to 70F, you get an RH of only 30%.  Also of interest: materials change temperature very quickly, in less than a day.  But it could take weeks or months to equilibrate to changes in RH (faster at higher temps, though.)

An important aspect of this new way forward is to realize it is based on algorithms, modeling, predictions and other scientific ways of guessing how materials are supposed to behave.  Since deterioration mechanisms of each material are complex and dynamic, it is difficult to get real info about damage to real artifacts.  This implies that the whole IPI scheme is a great hypothesis, and maybe the best one we’ve got, but there is precious little in hard data and proof.  For each of the three forms of RH/ temp deterioration, they’ve picked something to be the canary in the coal mine and then projected possible damage using math.

Biological: Measured by the Mold Risk Factor. Number 1-5, with anything above 1 indicating mold growth. Mold growth as a problem above 65%RH.  Spores won’t germinate below that, or if the temp is below 36F.  Germination happens in two days under favorable conditions (86F is ideal) but three years or more in marginal conditions.  Estimates came from published mold studies on stored food grains, and then mathematical modeling by Doug Nishimura of IPI and models from Stefan Michalski of CCI.  Higher temps also mean insects eat more and breed faster.

Chemical: Measured by the TWPI.  Higher numbers are better.  Temperature and humidity are the “speed controls” on natural aging, and they also combine with other agents of deterioration to accelerate decay.  “Deterioration is cumulative and progressive, and it doesn’t reverse itself when conditions improve.”  Example: milk doesn’t un-spoil when you put it back in the fridge after leaving it out for a few days.

Physical:  Measured by “dimensional change metrics” (shows up as EMC on software) with 2.5% or higher being bad.  They are estimating this aspect of deterioration from the behavior of an imagined block of wood from an “average species.”  Unclear what that means, really. 


PEM Preservation Environment Monitor.  Name of the IPI’s dataloggers.  Cost about $350 each in 2010.

Climate Notebook software sold that reads and interprets data from various loggers.

Preservation Index  (PI) Suggests, in years, how long it will take for a vulnerable organic material to become noticeably deteriorated under the conditions given.  Higher number (over 50) is good, with very good environments up around 200.

Time-Weighted Preservation Index (TWPI) A quantitative model of organic decay, tries to give an idea what is going on over a longer period with fluctuating conditions.  Higher number (over 50) is good, with very good environments up around 200.

Preservation Calculator: You enter temp and RH, it tells you the rate of natural aging from chemical change, and risk of mold for organic materials in general.  It gives you a PI number, too.

Dew Point Calculator: You can calculate temp, RH or dew point if you know two of the three.  Then it tells you the PI, mold risk and mechanical damage risk.  Using this helps know if your mechanical systems are humidifying or dehumidifying.  Also, IPI says keeping summertime dew points low is one of the most important factor of all in preservation.  If dew point is high outside, just cooling the air isn’t enough.  If you just cool it without taking water out, your indoor RH is way too high.

Equilibrium Moisture Content: Amount of moisture in a material at certain RH.  Above 2.5% in the Climate notebook interpretation indicates dangerous dimensional change.

Mecklenberg, M.F.  (2007) “Determining the Acceptable Ranges of Relative Humidity and Temperature in Museums and Galleries.”  Smithsonian Museum Conservation Institute 2007. 

2004 Smithsonian standards were around 45% RH +/- 8% and 70F +/- 4degrees.  Part one of the article is 57 pages on structural response to RH and part two is 29 pages on structural response to temperature.  In a nutshell, it seems that most materials are OK in the RH range of 30-60%, with 30-37% and 53-60% being cautionary regions.  For temperature, above 74F gets into increased chemical activity and should be avoided.  Down to around 55F is OK also, but below that we get into the glass transition temperature of many materials (acrylic paint has a Tg around 50F) and therefore risk brittleness and cracking.  A lot of this research involves paintings and wood.

Michalski, Stefan  (2007) “The Ideal Climate, Risk Management, the ASHRAE Chapter, Proofed Fluctuations and Toward a Full Risk Analysis Model”  Contribution to the Experts’ Roundtable on Sustainable Climate Management Strategies, April 2007, Tenerife Spain

“Proofed RH or T” is the largest fluctuation to which an object has been exposed in the past.  Risk of further mechanical damage from fluctuations that are smaller than the proofed value is low.  Ie, once the crack happens, it just opens and closes and doesn’t tend to get worse, especially if you stay away from those past extremes.  This is for mechanical damage.  Mold, chemical aging different.  When can repeated stress be handled indefinitely?  For tough materials, at about ¼ the stress that causes the damage can be repeated over and over OK.  For brittle stuff, about 1/10th.  So the safe zone is smaller than the proofed zone.  But even a little below the proofed value means there are many cycles needed.  Go straight to risk assessment based on past climate records.  Furniture, paintings other mixed collections susceptible to fracture from fluctuations: the worse their past the better their future.  “previous fluctuation history”  Response times of the objects?  Extreme fluctuations are sometimes shorter than the response time of the material.  Conservation treatments “erase the safety margin achieved by the fractures from historical conditions.”  Light fading risk goes down at low RH, but fracture risk goes up.  Higher temps reduce RH and help prevent mold, low temps reduce chemical self destruction.  Many collections survive well in non-ideal conditions, and many museums who claimed to reach the ideal didn’t.  People want a specification to deal with reality rather than the ideal.  He’s working with ICCROM on web-based manual for risk assessment.  This idea of proofed fluctuations is also going to appear in the 2011 ASHRAE chapter?  The part that is problematic in my mind regarding proofed fluctuations is new incoming artwork and artifacts whose proofed fluctuation may be smaller than that of the collecting institution overall?

Michalski, Stefan (1994) “Relative Humidity and Temperature Guidelines: What’s Happening?” Canadian Conservation Institute Newsletter. No 14. September 1994 pp. 6-10

Some basic concepts: low temps can cause some materials to get brittle, while high temps can promote chemical deterioration of many materials, especially acidic paper, nitrate/acetate films, celluloid and rubber.  Each drop of 5 degrees C doubles the lifetime of an artifact.  High RH promotes mold, corrosion, deliquescence, and promotes degradation through chemical self-destruction.  In most Canadian museums, the “proofed RH” seems to be +/- 25%.  Keeping within 20% +/- is suggested as reasonable, (although other articles suggest that 20% is too great, and maybe 10% is more appropriate?)  Useful table “Effect of Incorrect RH and Incorrect Temperature on Museum Materials”  The Abbey Newsletter, Vol 18 No 8 Dec 1994 has a review of this article by Ellen McCrady, a book and paper specialist.  She said the article was accurate for paintings and objects, but weak in the realm of paper and photos. 

Ogden, Sherelyn. (2007) NEDCC Preservation Leaflet 2.1 Temperature, Relative Humidity, Light and Air Quality.  Basic Guidelines for Preservation.  North East Document Conservation Center.

Deterioration doubles for every 18 F temperature rise.  Libraries and archives prefer RH 30-50%, the lower end of that the better.  Excessive fluctuations in RH can lead to dimensional changes, cockling, flaking inks, warped covers, and cracked emulsion on photographs.  She does not specify what fluctuation over what time period would be excessive.

Pacifico, Michele F and Thomas Wilsted.  (2009) Archival and Special Collection Facilities: Guidelines for Archivists, Librarians, Architects and Engineers.  Chicago.  Society of American Archivists.  

Padfield, Tim (2009) “Lecture on Archive Environmental Standards.”

BS5454:2000 is the British Archive Standard, is difficult to enforce and seems rather arbitrary.  16-19C for handled stuff, 13-16C for stored stuff, and an RH 45-60% with a 5% fluctuation.  Science still cannot measure very slow deterioration.  Interesting historical and archaeological examples:

St Catherine’s Monastery in Sinai Egypt dates back to the 6th century.  RH 10-30%, temp 8-30C

Desert of Eastern Turkistan, 3rd century papers buried in sand in an abandoned building.  RH 30-50%, temp -10 to 25C

Padfield, Tim (2005) “How to Keep for a While What You Want to Keep Forever”

Respected scientist and consultant in the field of museum climates.  “The standard we have in conservation are particularly unconvincing and the evidence that supports them is shaky and controversial”

Padfield, Tim (2004) “The Preservation index and the Time Weighted Preservation Index”

Article about the Image Permanence Institute’s standards for preservation storage environments.  “Preservation index” links temp/RH to chemical deterioration (as opposed to biological or mechanical) based on a study of the deterioration of a common photographic film, cellulose acetate, through hydrolysis.  “Time Weighted Preservation Index” is a way to indicate the situation taking the variation of the room over time.  Padfield explains the chemistry (law of mass action and Arrhenius law) for the degradation of the cellulose acetate reaction and how both temperature and RH impact it.  He has a slightly different interpretation of what is going on chemically than IPI does, but doesn’t seem to quibble with their product.

Rawlins, (1942) “The Control of Temperature and Humidity in Relation to Works of Art.”  The Museums Journal (Museums Association) 41: 51-90

Very early and influential text setting the standard at .  “Target “60F 60% 

Reilly, (2008) “Specifying Storage Environments in Libraries and Archives.”  From Gray Areas to Green Areas: Developing Sustainable Practices in Preservation Environments, 2007, Symposium Proceedings by the Kilgarlin Center for Preservation of the Cultural Record, School of Information, The University of Texas at Austin.  Published online: September 2008,

“One cannot manage what cannot be measured”  In contrast to some other articles (chicora 1994)  he advocates that temperature matters more than RH.  Like Michalski’s articles, Reilly explains the risks in terms of biological, chemical and mechanical damage.  This model is really helpful.  Biological damage (especially mold) happens with high RH, usually above 65 for a few days or more.  Chemical damage is accelerated with heat and moisture, and examples include corrosion, yellowing, embrittlement, and fading.  For chemical change, short term fluctuations don’t matter much.  Mechanical damage mostly relates to hygroscopic materials, and the damage would be cracks, deformations, delamination etc.  High RH causes hygroscopic materials to swell and soften, while low RH causes them to shrink and become brittle.  For avoiding mechanical damage, you need to stay between 20-70% RH and keep excursions outside that range as short and infrequent as possible.  Seasonal extremes are worse than short-term events because materials take considerable time to equilibrate…weeks or months, not hours or days.  Room temperature is too warm for ideal preservation of many organics, paper, leather, dyes, film, etc.

Ryhl-Svendsen, Morten, Lars Aasbjerg Jensen, Poul Klenz Larsen and Tim Padfield.  (2010)  “Does a Standard Temperature Need to be Constant?” Presented at the Going Green conference, British Museum, April 2009

Article argues for allowing temperature fluctuation greater than traditional standards allow while keeping RH controlled.  Uses three institutions in Denmark as an example.  Royal Library in Copenhagen is about 65F and 50%, permanently air conditioned.  Arnamagnaean archive in Copenhagen University has buffered conservation heating, which allows the temperature to fluctuate as the outdoor temp fluctuates but keeps a steady RH.  Museum store in Ribe, Denmark has dehumidification only, with temp of 50-60F and a steady RH as long as the equipment is functioning properly.  When it does not work, RH reaches dangerous levels.  Ventilation is an issue, and the authors recommend recirculation of the inside air through a filter.

Sebrera, Donald K.  (1994) “Isoperms: An Environmental Management Tool”  Washington DC: The Commission on Preservation and Access

Credited with being the first person to explore preservation, temp and RH relationships using a “kinetics model based soley on theory and assumptions about the likely activation energies of decay reactions.” (quote from the IPI Step-by-Step workbook)

Thompson, Garry. (1978) The Museum Environment  Butterworth Heinemann

Until recently, the contemporary standard for museum environment.  50% RH +/- 5% and 70F +/- 3 degrees.  Allowable fluctuation apparently based on the switching differentials of the best available HVAC systems of the 1970’s.  Part of the reason this book is so influential is that the author worked for the Scientific Department of the National Gallery, London and examined the available scientific evidence.  He also was aware that there was little proof for the limitations, and qualified his statements carefully.  He chose 55% largely as a midpoint between 70% risk of mold growth and the then-prevalent notion that below 40% organic materials become brittle 

Weintraub, Steven.  (2006) “The Museum Environment: Transforming the Solution into a Problem.”  Collections: A Journal for Museum and Archives Professionals Vol 2. No 3 (February 2006) pp 195-21