Tag: Steven Weintraub

Relative being the key word in this talk, Steven Weintraub of Art Preservation Services, Inc., presented a checklist of critical thinking when making decisions about relative humidity (RH) microclimates for collections.
Question the accuracy of your RH measurement
Weintraub points out it’s really easy to be 5% off  on measuring RH for a myriad of reasons including sensor locations relative vent locations, drift in the measuring equipment. While 40% to 60% is the usual goal, a conservator has to ponder how comfortable are you with 35% to 65%?  Weintraub admitted those extremes make him less confident for preventive conservation; microclimates can be the answer when an object requires tighter control.
The talk ended on this accuracy theme as well. While technology has come to RH measuring systems such as blue tooth systems so the case no longer has to be opened, accuracy remains in issue. Before setting up an exhibit, compare all the meters so to have at least an internal standard for readings.  Calibrating the meters before exhibitions is ideal, of course but not always feasible.  If there is a large discrepancy in RH readings between the loaning institution and your institution, it might be worth having a conversation about calibration methods.
To seal or not to seal a case
Weintraub recounted the common reasons for not sealing a case: Avoid trapping off-gassing; gallery climate control is adequate; it’s harder and more expensive to construct an airtight case. However hindsight is harder to manage. It’s harder to retrofit a leaking case and make it air tight after the fact when too much dust is collecting on the objects or other problems occur. Thus it’s best to start with air tight cases and loosen if needed.  Hence whether intentional or not, sealed cases are microclimates.
Microclimates: Active control, Passive control, or Nothing
Weintraub recommend building all cases to have the provisions for at least a passive RH control system regardless. Again the theme of enabling flexibility and avoiding retrofitting later applies. Building space for silica gel trays and not using it is easier than retrofitting the case later.
What is the rate of leakage for the case is the most important question for microclimates. The leakage rate will determine if a passive control is adequate or active control system is needed. Weintraub noted, no silica gel system in the world is adequate for a highly leaky case. Nominal leaking from a tight system then begs the question about why an active system is needed.Leakage assessment can be easily accessible. Weintraub feels it’s important and empowering for institutions to be able to conduct their own leakage rate tests. It will enable identifying when repairs are needed under service contracts and also make more informed choices about the steps needed for microclimates. A caveat on interpreting leakage rates when you’re shopping for cases No standard protocol exists for determining leakage rates; so manufacturers reported values are hard to compare. Leakage rates change over time as materials age and warp
 Creating your own leak detector
Weintraub shared two easy ways to have your own leak detection system. Cans of dust-off contain small amounts of refrigerant.  A refrigerant detector can be easily purchased from HVAC suppliers for about $500; the detector is akin to a Geiger counter. It’s a qualitative tool that helps locate the leaks. The second leakage assessment choice is monitor carbon dioxide levels. The carbon dioxide level in the case increased above ambient levels (600 to 2500 ppm) and use a meter installed in the case to monitor the change in carbon dioxide levels.  Let the case reach equilibrium at before starting the leak test. Weintraub and students at the NYU conservation center are currently examining how long it typically takes to reach equilibrium. Weintraub likes to run his leak tests for 3 days. Basically it’s calculating the rate of loss of CO₂ Thus the difference in CO₂ measurements over the time period.  Close to 0 for the rate means success as there is minimal leakage. A large rate indicates an issue. At that point, consider looking at the half-time decay, how many days it takes for CO₂ levels to drop 50% in the case.
How much silica gel?
  Answer: Leakage rate * number of exhibit days* buffering capacity of silica gel at your target humidity levels= weight of silica gel.
You can examine compare different silica gel types for your scenario as some silica gels perform better at high humidity and others at low humidity. For a maintenance-free case, Weintraub’s rule of thumb is double the exhibition quantity of silica gel.   Another silica gel tip is to mix silica gels at different humidities to get the target humidity such at mixing 55% and 40% RH gel systems to get a target of 50%.
Also, mind the air gap in the case. An air gap is needed to make sure air flow is adequate in the case to get the benefits of silica gel actually reaching the collection objects.
Lastly, we as conservators need to do a better job of sharing our learning and experience about microclimate to develop a collective pool of knowledge
Weintraub’s article on Demystifying Silica Gel is available on Art Preservation Services website along with some of his work on LED.