Contributed by John Bergman-McCool
Back in March I wrote a blog post summarizing efforts to rid collections objects of mold and salt uncovered during inventory and rehousing. We identified and isolated affected objects and cleaned them by dry brushing and vacuuming. The cleaned objects were rehoused in archival boxes that included a sachet of silica gel. The purpose of the gel is to reduce relative humidity (RH), thereby robbing mold and salt of the environmental conditions necessary for their growth. To better understand what the environment is like inside the boxes, we are monitoring their temperature and relative humidity with two data loggers. One is placed inside a box without silica gel and one is placed inside a box with silica gel. These conditions will be compared against a data logger that is recording general conditions in the basement not far from where these test boxes are located. We will be watching these data loggers over the coming year, but we already have some interesting results.
First, the boxes are working well as a buffer against relative humidity cycles. The graph above shows RH and temperature for the month of April; the basement is shown in red and the boxes with and without silica are blue and yellow, respectively. In April the RH in the basement was quite volatile. However, the RH inside the boxes is remarkably tranquil in comparison. The boxes are exhibiting small daily shifts of 1 or 2%, which is acceptable. Keeping RH from shifting dramatically is an important factor in collections care. Organic materials such as basketry, bone, and wood are hygroscopic, meaning that they can absorb and release moisture in the air. Rapid and large changes in RH can cause organic materials to swell and contract leading to damage such as cracking or delamination. It is best to keep collections from experiencing RH shifts exceeding 10% over a given month and on that count the boxes are doing a great job. As they are found, the most sensitive organic collections are being moved to another part of the museum that has a better environment.
The National Park Service recommends creating a layered approach to collections storage. Every enclosure within museum storage can act as an environmental buffer. The first enclosure is the building itself. It may seem pretty obvious, but keeping collections inside a building greatly reduces the effects of environmental factors. The same is true of every subsequent layer of enclosed storage. Here at the Peabody Institute we have wooden storage bays that, when closed, serve as another layer. The archival boxes act as a final layer.
Interestingly, the basement seems to be effective at buffering daily temperature cycles. The temperature in the basement has been hovering around 70 between February and June leaving little for the boxes to mediate.
The second finding of note is that the sachets of silica gel were spent faster than anticipated. As mentioned above sachets of silica gel were placed in the boxes with cleaned objects. The gel, in solid pebble-like form, starts out orange and as it absorbs water it changes to a deep blue. The expectation was that the gel would keep the RH at a reduced and steady level. The graph above shows that the silica gel was keeping relative humidity lower than that of the box without gel, but it is only a matter of a few percentage points. Most likely the boxes are not well enough sealed for the silica gel to more significantly moderate RH levels. The silica was active from mid-February until mid-April (see star on graph) when RH graphs inside both boxes started to match almost perfectly. A visual inspection in June indicated that the gel was spent. We replaced the silica in mid-June and it was spent within two weeks given the higher RH levels generally in the basement.
Our data shows that the boxes are acting as a significant buffer against potentially damaging cycles of increasing and decreasing RH levels. For now, we are forgoing replacing spent silica gel. Later in the fall we’ll see how the archival boxes work with our dehumidifiers at keeping mold and salt inducing RH at bay.