Hi all, Thanks for all of the responses that I have been getting! I'm including a bit more information and what we are planning on doing to test it.
The cases were custom made and vary in size. The bonnets are made of UV filtering acrylic, the back wall and decks are made of Sintra and birch plywood covered with dyed linen, and the bases are made of plywood coated with 2 coats of VOC free latex paint. There is no lighting inside of the cases. All lighting for the exhibition is provided by track lighting with LED lights. The temperature both inside and outside of the cases appears to be consistent, with the temperature inside the cases being buffered from more abrupt changes in temperature. I am unsure of the air exchange rate in the cases. The RH dropped very quickly once the cases were closed. The RH was originally 41% and it steadily dropped to 35% over 5 hours where it has remained for the past month and a half. Cases with mixed media were given two sachets made of linen which were hung in the recess of the back wall of each case - one of activated charcoal and one of ArtSorb preconditioned to 50% RH. Unfortunately, we used our entire supply of activated charcoal, so there is none left in the lab to test. The RH test that I performed was as follows: I placed a small amount of the ArtSorb into a sealed container and let it acclimate overnight. I then took a reading of the ambient RH with a digital hygrometer - the reading showed the ambient RH in the lab was 35%. I removed the seal covering a small hole in the container and inserted the hygrometer. The RH instantly shot up to 52%. I received consistent readings when I checked later in the day and when I repeated the test a week later. We're planning to collect some data over the next week by removing the charcoal from one case, adding more silica gel to another, and keeping a third as a control to see what can be done. We're fairly certain that it's the activated charcoal that is causing the drop in RH. Thanks again for all of the input! Best, Liz ------------------------------ Message: 2 Date: Tue, 18 Jul 2017 11:40:35 +0000 From: "Shashoua, Yvonne" <[email protected]> To: "'[email protected]'" <[email protected]> Subject: [Consdistlist] Low RH in case with activated charcoal and silica gel Message-ID: <[email protected]> Content-Type: text/plain; charset="iso-8859-1" Although we conservation professionals use industrial adsorbents to remove a specific vapour or gas from showcases or storage areas, their chemical and physical structures enable them to adsorb families rather than specific pollutants. You are using silica gel to adsorb water vapour but it is also an effective adsorbent for formaldehyde and acetic acid. Likewise, activated charcoal has a surface area of up to 1500 m2/g with pores between 10 and 30 ? in diameter). Molecules are held within pores by weak physical bonds. Activated carbon is non-polar and therefore has a high affinity for organic molecules. Although it has a low affinity for water, its ability to absorb liquid water was determined and published as early as 1949 http://pubs.acs.org/doi/abs/10.1021/ja01170a051 I suspect that the activated charcoal is contributing to the reduced RH in your showcase. Removing it and observing the resulting RH will confirm or refute this suggestion. Yvonne Shashoua Senior Researcher National Museum of Denmark email: yvonne.shashoua-at-natmus.dk ------------------------------ Message: 3 Date: Tue, 18 Jul 2017 10:52:36 -0400 From: Jerry Shiner <[email protected]> To: [email protected] Subject: [Consdistlist] Low RH in case with activated charcoal and, silica gel Message-ID: <[email protected]> Content-Type: text/plain; charset=utf-8; format=flowed Low RH in case with activated charcoal and silica gel Liz Did you check the rh of the activated carbon? How much carbon is there? How large is the showcase? What else is in the showcase? How long did it take for the rh to change? Was it ever satisfactory? Do you have a graph tracking rh / time? It seems entirely plausible that the carbon was shipped in a dry (or very dry) state, and that it has ample capacity to absorb lots of moisture from the showcase (including the preconditioned silica gel). Answering some of my initial questions might provide clues. I also have questions about how you tested your silica gel. Also note that to test silica gel, best results will be found by sealing your sample of sg and your rh probe in an impermeable bag or small box, and then be certain to give the hygrometer/silica gel/air combination time (hours to days) to "settle". This is important as it is very possible that the rh of the exterior of the sg grains differs from the interior, and it may take some time to equalize. A further comment: For your application, sg is not the best product - it is superior when creating or buffering dry conditions (under 40%). A sg-based buffering compound like Prosorb has far more capacity in the middle range (rh 40% to 60 %). js -- Jerry Shiner Keepsafe Microclimate Systems www.keepsafe.ca +1 416 703 4696 [email protected] Skype: jerry.shiner Specializing in the design, procurement, and installation of environmental control systems for museum and archives ****** Unsubscribe by sending a message to [email protected] Searchable archives: http://cool.conservation-us.org/byform/mailing-lists/cdl/
