Too little gel will require more frequent replacement, and an excess of gel will extend the time before replacement.Īn example calculation for a storage box could be:Ī container that is L = 30 cm, W = 20 cm and H = 10 cm would have a volume of 6000 cm 3. Many cards change from blue to pink as the humidity starts to climb. The volume of moisture sorbent is not critical for storing metals because a humidity-indicating card (available from museum and conservation suppliers) placed in the container will gradually show increasing humidity and will signal when the silica gel needs to be regenerated or replaced. To keep the calculation simple, the volume of the objects and the volume of the silica gel can be ignored. Using metric values and measuring your lengths in centimetres will make the calculations easier because you can divide the calculated volume by 1000 to get the number of litres. For cylindrical containers, the formula is the height multiplied by the radius (half of the diameter) squared, multiplied by π (pi), which is (H × r 2 × 3.14). If the moisture sorbent comes pre-packaged, then a certain number of units will be needed.Ĭalculating the interior volume of a square or rectangular box is done by multiplying the length by the width by the height (L × W × H). Calculating silica gel quantityĭetermining how much of a moisture sorbent to use is not a difficult calculation. Further information on the use of silica gel to control humidity can be found in Tétreault and Bégin (2018). To keep things simple, this Note will use regular density silica gel with a mesh size of 6 to 12 as an example of a moisture sorbent that has been used in museums for many decades and that performs well at an RH below 45%. The gel Artsorb should not be used, however, since it can corrode metals (Robinet 2007). More recently, there have been other products, such as aluminum silicate clay, molecular sieves as well as various formulations of silica gel (Art-Sorb, PROsorb and RHapid Gel), that have provided varying degrees of performance. When sealed in packets of polyethylene non-woven sheet (such as Tyvek) or sewn into unbleached cotton bags, silica gel still provides a cost-effective moisture sorbent alternative that is readily available from museum, office and scientific suppliers. Traditionally, the most common moisture sorbent used in the museum community has been silica gel (amorphous silicon dioxide). This type of storage is often not suitable for non-metallic materials. Whatever system is chosen, it should be designed for the storage of metals over many years with no need for frequent maintenance. The idea is to establish dry conditions as a microenvironment in a container that will slow down or halt active corrosion initiated or accelerated by humidity in the air. Nevertheless, there is a solution for smaller metal objects that consists of a plastic box and a dry moisture sorbent. As a result, few institutions can provide a collection space specifically tailored for the dry storage of metal objects. Generally, only a part of a collection is made up of metal objects of those, only a few will be at risk of ongoing corrosion. However, RH, the key factor, can be reduced for museum objects made only of metal (for more information, consult CCI Note 9/2 Storage of Metals). Not all of these factors can be controlled, such as the type of metal in the object or the amount or type of corrosion that has occurred in the past. The extent to which this happens will vary depending on several factors that include the type of metal, the amount and type of previous corrosion, the presence of surface contamination like dirt or salts and the relative humidity (RH) of the surrounding air. Many metal objects in museum collections will have already corroded due to exposure to moisture before becoming a part of the collection.
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