D. Gilroy and I. M. Godfrey
The most popular notion of a time capsule is something resembling a giant vitamin pill crammed with photographs, coins and assorted memorabilia, which is either buried or placed in the foundations of a building for a period of 100 years. This method, or variations of it, were employed in Victorian times. Unfortunately the use of incompatible or unstable materials and poor packing techniques have meant that little material was recovered in good condition from time capsules of that era.
As time capsules are sealed containers, deterioration may be accelerated, particularly if unfavourable microenvironments develop in the capsule. Unfavourable conditions may arise if:
- stored materials deteriorate;
- capsules are not made from appropriate materials or are not well sealed; and
- unstable storage conditions lead to fluctuations in internal temperatures and relative humidity levels.
The lack of air exchange with the surroundings means that if for example, acidic vapours are released by an object as it ages then the rest of the interred objects will be subjected to an increasingly damaging environment. Some wood, paper and modern materials such as polyvinyl chloride behave in this fashion.
Changes in external conditions also affect the internal environment unless the storage conditions of the capsule are well controlled. A large drop in temperature, for instance, would produce an increase in relative humidity, a condition which may lead to condensation in the capsule. The growth of mould on organic objects and the corrosion of metals would be encouraged under these conditions. Conversely, if the temperature increases too much then the resultant low internal relative humidity may lead to damage such as the embrittlement of paper and wood and crazing or cracking of paint layers. Internally and externally insulating the time capsule will minimise relative humidity fluctuations.
Relative Humidity and Site Selection
The factors affecting relative humidity and strategies that may be used to control it are discussed earlier (see the chapter Preventive Conservation: Agents of Decay).
In order to ensure the long-term survival of materials placed in time capsules, pack the capsule when the ambient conditions are cool and the relative humidity is low. While these conditions are usually better for the long-term preservation of most material types there is a risk of desiccation of susceptible materials. As long as the capsule is stored in a stable environment then internal fluctuations will be small. If the capsule is packed on a day of very high relative humidity then that moist atmosphere and moisture laden materials will be sealed in the capsule - a recipe for disaster unless moisture-controlling agents are incorporated in the time capsule.
In selecting the interment site, consider:
- the temperature range associated with the site;
- the nature of the site in which the capsule is to be buried: and
- the likelihood of the site being disturbed before the time capsule is due to be opened.
The burial site may be below ground, in a wall or building or even on the ocean floor! As each site will have its own set of unique environmental parameters, the nature of the site necessarily dictates the type of construction materials that should be used for the capsule. Chemical features of the soil, depth of the water table and drainage are some of the factors which must be considered for a land site.
If the capsule is to be buried below ground then an appropriate stainless steel container is recommended, either sealed with an o-ring or with a welded lid. This type of container will minimise corrosion and leakage problems. Depending on the size of the capsule such a container could be very expensive. Alternatively if the capsule is to be placed in a crypt within a building then cheaper materials such as polyester powder-coated iron could be used.
Factors affecting the construction materials and the type of capsule include:
- length of time the capsule is to be buried;
- volume of material to be included in the capsule;
- location and nature of the burial site; and
- available funding.
For burial below ground the following materials are suitable:
- certain grades of stainless steel;
- copper and its alloys; and
- high density polyethylene (HDPE) pipe.
Materials suitable for above ground storage include:
- galvanised metal;
- polyester powder-coated iron; and
- enamelled metal.
Capsules that are either seamless or of welded construction are preferred. HDPE provides a cheaper, though less durable material for underground burial. This material may be sealed by hot-air welding, thus avoiding the use of adhesives which would be prone to breakdown in the soil. HDPE is not recommended for above ground use unless it is protected from weathering agents such as the sun and rain.
Do not use polyvinyl chloride tubing or iron materials that are not galvanised, enamelled or polyester coated as they are all prone to deterioration.
The lid on the capsule should either be welded or have an o-ring seal on the lid section and be constructed so there is an even pressure exerted on the lid. This may be achieved by having bolts evenly spaced around the lid or by using a screw-on lid. If bolts are used to secure the lid, locate them on the outside of the o-ring seal so that the seal is not compromised. An airtight seal is important if moisture, dirt and insects are to be excluded from the capsule.
Silicone sealant can only be used externally as the fumes given off as it cures could harm the contents of the capsule.
Insulate the capsule if it is to be interred above ground. The effect required is similar to a thermos flask where the box in a box principle operates with a key aim being to minimise changes in internal conditions. This may be achieved for example, by placing the capsule inside an insulated box which is then stored in the centre of an air conditioned building. If interred below ground the surrounding earth provides the insulation.
Materials for Inclusion in the Capsule
It is most important to think long and hard about just what is to be included in the time capsule. Factors to consider include:
- durability of materials;
- quantity and size of objects;
- compatibility of materials; and
- likely future availability of technology to replay included objects such as audio or video materials.
Think carefully about the objects to be placed in a time capsule and choose objects that most appropriately represent the present time. Avoid things that are likely to survive in other places such as in newspaper archives for instance.
Some materials are more prone to decay than others. Only include unstable objects in the capsule if they are highly significant and if they can be effectively isolated from the other objects in the capsule and treated to enhance their longevity (via oxygen exclusion for instance). The most suitable objects are those that are self-contained and self-explanatory. That is, they don’t need technology to either read, interpret or use them.
Seek the advice of a conservator regarding objects for inclusion in a time capsule so that unsuitable materials may be excluded and any materials that require treatment to increase their chances of surviving the burial period can be identified and treated prior to their inclusion in the capsule. Newspapers for example, are notoriously poor in quality and should be double packed to ensure that there is no contamination of other materials. Wrap them in alkaline-buffered paper and encapsulate in polyester to minimise contamination risks. A better alternative is to use photocopies of newspaper articles made using archival quality paper.
Where possible, use archival quality papers for included documents. As for newspapers, if original documents are printed on poor quality papers, copy them onto alkaline-buffered paper for inclusion in the capsule.
Many audio-visual materials, such as video and cassette tapes, are inherently unstable and may not last more than a few decades before suffering serious deterioration. If argon or nitrogen flushing is not possible then vacuum packing or storage of these materials in a sealed container with Ageless® oxygen scavenger is recommended. Their inclusion must also be weighed up against the likely availability of the equipment needed to replay them when the time capsule is opened. Will video technology still be available for playback?
Do not include synthetics materials such as PVC, rubber and plasticised materials. Plastics considered safe for use in time capsules include polyesters, polyethylene, polypropylene, polycarbonate and polystyrene. As it is sometimes difficult to differentiate between these synthetic materials seek professional advice before making a decision to include them in the capsule.
Photographs are popular inclusions in time capsules. Colour photographs are less stable than their black-and-white counterparts. Cibachrome processing is recommended if colour prints are to be included. Use archival standard processing techniques to increase the longevity of images that are included in the capsule. Do not stack photographs on top of each other without a sheet of polyester film between them. This will stop the photographs from sticking to each other (see the chapter Photographs for more guidelines).
If textiles are included in the capsule, make sure they have been cleaned, are free of insect and microbiological infestation and are conditioned to the humidity of the capsule. Wrap them in unbuffered, acid-free tissue.
Packing the Capsule
Rather than specifying a long list of ‘dos and don’ts’ regarding packing, it is better to stress the importance of common sense and care when packing the capsule. Where possible, either individually wrap all items or at least group similar materials together and wrap these as one (Figure 5). For example keep paper apart from metals. This can be done most cheaply by using either heavy-duty polyethylene bags or polyethylene containers. Heat seal the bags. Put heavy items on the bottom of the capsule.
As mentioned earlier in this section, pack the capsule when the temperature and relative humidity are at the most appropriate levels for the materials that will be included in the capsule.
Avoid folding objects, as it introduces stresses into them. If materials such as papers and textiles cannot be laid flat then roll them for storage. It is wise not to cram too many objects into the capsule.
Flushing the capsule with an inert gas such as nitrogen or argon or using oxygen scavengers such as Ageless® will help to prevent deterioration of the contents by excluding oxygen. To avoid damaging the capsule contents, use a long pipe and trickle feed system to introduce the gas. In this way the contents will not be subjected to a sudden drop in temperature and possible condensation. Argon, although more expensive than nitrogen, is preferred as it is heavier and less likely to be lost from the capsule over time. Add silica gel or zeolites to the capsule if low relative humidity conditions are required.
Note that if the capsule is flushed with an inert gas, also flush any materials that are sealed in polyethylene bags with the same gas. If this is not done deterioration due to oxidative processes will continue in the sealed bag, defeating the purpose of flushing the capsule with the gas.
Label objects included in the capsule and unless the object is a document or something similar which tells its own story include some information relevant to the object in question. Use archival quality paper and card for object labels, write on these with non-water soluble inks and attach the labels to the objects with stable tapes or cords or include them in sealed bags with the objects.
Lodge details concerning the location, contents and future plans for the time capsule with the appropriate authorities. This may be a local government agency, a museum, library or historical society. Provide this information to more than one organisation.