As you can see on table 1 of the PRO SORB technical information, conventional silica gels are quite useful in the range between 0% and 40% RH as well as above 75% RH only. In the intermediate region (40 75% RH), ideal for most works of art, PRO SORB and ART SORB are far superior to conventional silica gels.
PRO SORB and ART SORB are excellent in stabilizing relative humidity even under fluctuating temperatures. For organical materials (wood, textile, painted metal...), ideally both, relative humidity and temperature, should remain constant.Here you find:
Due to the many factors involved such as leaks in the case/crate, sunlight, temperature extremes, etc. it is difficult to predict the required amount.The minimum figures given in the ART SORB technical information leaflet refer to the frequent case that in a room without temperature or humidity extremes a well sealed showcase has to be kept in a narrow range of relative humidity. To determine the exact amount of ART SORB you will always have to make your own consideration. If the air exchange rate of the showcase is known, theoretically the amount can be calculated. But even this air exchange rate can change due to handling of the showcase. In practice you have to start with a certain amount of ART SORB and you should monitor the relative humidity in each showcase (we offer small thermohygrometers for this purpose). Last but not least: it is always a good idea to test the climate inside the case before you put in the art objects!
Several factors may increase the necessary amount of ART SORB in a showcase:
The moisture adsorption capacity of wood should not be underestimated: In the range between 40 and 60% RH, 6 kg wood have a moisture adsorption capacity similar to 1 kg of ART SORB. Therefore, wooden in-case constructions can seriously interfere with ART SORB. However, the speed of adsorption/desorption of wood is very low. A 20 mm thick wooden board has a 50 days equilibration halftime (Padfield, online). Thus, it´s practically impossible to condition the wooden constructions to the desired relative humidity before the exhibition. If you cant avoid wooden constructions in showcases completely, try to interleave aluminium barrier films between wood and decorative fabric. Increase the amount of ART SORB.
It is recommended to keep some additional ART SORB on site. This will serve as a "swap" when a particular cases ART SORB needs to be re-conditioned.
The more contact ART SORB has with its sealed environment the better it is able to condition the surroundings within a minimal time. The air-gel interface can be maximised by spreading the bead type as thoroughly and evenly as possible, for example in shallow flat trays (maximum depth 3 cm).
If you prefer to work with cassettes it sometimes makes sense to use two half-size cassettes instead of one full-size cassette or to add ART SORB sheets for short response time.
Passive climate control needs well sealed cases made of glass, metal and other substances impermeable to moisture. In a medium-sized showcase the gaps at the top and the bottom should be smaller than 0,3 mm (thickness of heavy paper) in order to reach an air exchange rate below 1 per day. This should be considered the absolute minimum for successful passive climate control. To create the appropriate climate is much easier in horizontal showcases than in high vertical ones. If you want to know more, Stefan Michalskis brilliant article (s.b.) will tell you everything about it.
In most cases, the ART SORB material is hidden in the base of the showcase, thus good air exchange between base and case is essential. Ideally, there should be slits at least 20mm wide all around the bottom of the case. Case bottoms or perforated metal sheet are a good option as well if the holes are large enough (5mm or more).
Make sure that you can change the ART SORB without rearranging your display! Flat drawers or flaps accessible from the outside are ideal. Please remember that ART SORB should not be in direct contact with oxidizing metals like iron, aluminium, zinc or brass.
A comment on well sealed cases: Some case materials like wood - but also the displayed objects themselves - might out-gas chemical compounds like acetic or formic acid which might shorten the lifespan of your precious art objects. The more a case is well sealed, the better its compounds are able to concentrate and accumulate. We offer a choice of pollution scavengers like activated charcoal to counteract this chemical degradation.
Usually, ART SORB is delivered pre-conditioned at 50% RH ±3% RH. Any other value is available for a moderate supplement. In order to avoid any damage to your art work you should always control the conditioning of your ART SORB before use. To determine the actual conditioning of the ART SORB material please use a hygrometer which is known to be well calibrated and precise. We consider the Testo 645 with high precision probe best for this purpose and use it for our regular controls. You can use it for readjustment of your hygrothermographs as well.
If you wish to measure the actual conditioning of your ART SORB material enclose it together with your hygrometer in a non-permable bag, keep it in a room with constant temperature and read after 24 h.
The Testo 645 with high precision probe or LUFFT control hygrometer 5804 (on photo) can be used for this purpose
If the set point of your ART SORB proves to be too low or to high for your application it will have to be re-conditioned. The rules below will help you and will guarantee a long lifespan to your ART SORB.
1) The simpliest method (but time-consuming) is to put the ART SORB into a room which is known to have the desired relative humidity and let it reach equilibrium. A humidity chamber is perfect, too.
2) Sponge method: by placing a moist sponge or cloth with ART SORB in a sealed case, water can be adsorbed and the weight checked or continuosly monitored until appropriate weight is reached.
3) Direct application of water by misting or a spoon. Dry beads may crack during this process, but this does not change ART SORBs adsorption capacity. Cracking is negligible above a set value of 40-50% RH.
Bead type: a cycle of misting and stirring the beads to promote adsorption should be employed.
Cassette type: care must be taken not to moisten the cardboard. The best method is to humidify a small spot on the PE/PP-mebrane with very little alcohol or isopropanol by means of a slightly wet kleenex. Then the distilled water passes through the membrane easily. Applied with a spoon the water normally doesnt even touch the cardboard. This applies to the yellowish cassettes. Our new grey cassettes are waterproof. You can moisten them like PRO SORB cassettes, see PRO SORB directions.
Sheet type: a misting and a waiting period should be used to allow adsorption followed by checking with a hygrometer.
1) Again, a room with the desired relative humidity or a humidity chamber is a simple but time-consuming method.
2) ART SORB can also be dried directly in an oven, but care must be taken not to exceed 65°C. This method won't be quicker than the desiccant-bags following up.
3) The best method is to place ART SORB in a non-permeable bag and to add dry desiccant bags. It is very time-effective and works well for beads and for cassettes. A dry 250 g desiccant bag takes up about 48g of moisture between 0% RH and 40% RH. The desiccant bags can be dried in an oven at 130°C (see page "silica gels...").
In some cases it may be possible to leave ART SORB in the showcase and to correct the RH-value of the showcase simply by adding small amounts of desiccant bags - the easiest and least time consuming procedure.
ART SORB contains silica gel and lithium chloride. It is harmful if swallowed and irritates skin and eyes. Dont inhale eventual dusts. Further informations see Material Safety Data Sheet..
The water adsorbed by ART SORB is present in a liquid phase on the surface of the particles. Direct contact with metals like iron, zinc and aluminium may cause oxydation. Therefore ART SORB should not be in direct contact with art objects, especially since the liquid is containing chloride ions. The same characteristics apply to dusts. Try to avoid dusting by careful handling and keep them away from your art objects. Dont touch ART SORB and art objects in turns in order not to transfer dusts to them. ART SORB should not be used in an environment with rapid air movements (>1m/s).
Kamba, Nobuyuki: Measurement of the Dimensional Change of Wood in a Climate Case. ICOM Committee for Conservation. 10th Triennal Meeting 1993, Washington, p. 406-409.
Padfield, Tim: An Introduction to the physics of the Museum Environment (On Line). http://www.natmus.dk/cons/tp/index.htm
Toishi, Kenzo: A note on the movement of moisture between the components in a sealed package. Studies in Conservation 39, 1994, p. 265-271.
Michalski, Stefan: Leakage Prediction for Buildings, Cases, Bags and Bottles. Studies in Conservation 39, Nr. 3, 1994, p. 169-186.