Bottles and jars form the basis of many collections. As most bottles are made from glass the aspects of deterioration, care and conservation outlined earlier in this chapter apply to these objects. It is most important to be aware of the history of collected bottles, as the burial environment from which they were recovered will influence deterioration processes.
In addition to the usual factors which affect the deterioration of glass, internal stresses will be present in hand- or machine-blown glassware. This is due to differences in cooling and shrinkage rates between the thin and thick sections of the molten glass vessel. Exposing bottles to sudden changes in temperature may upset the balance of these internal stresses, resulting in the glass fracturing. For this reason always allow glass to adjust gradually to new environmental conditions. Such allowances should be made following excavation, washing and movement from one climatic region to another.
Weathered glass attains a variety of colours and textures depending on the environmental conditions to which it has been exposed. Bottles that have been in a moist environment will become iridescent or silvery and the surface may exfoliate. Glass buried in an alkaline environment such as found in guano sites for example is characterised by pitted, opaque surfaces.
Clear glass exposed to sunlight for a long time may become purple, indicating that manganese was used in its manufacture. Some clear glass will become amber, an indication that selenium rather than manganese was used as the decolourising agent. As selenium was used between 1915 and 1930 and manganese was used predominantly between 1880 and 1915, these forms of deterioration are important as dating indicators.
As long as they are placed in a suitable environment, most weathered glass objects will not deteriorate further after soluble salts have been removed from them.
During and after excavation, do not allow wet bottles to dry. Keep plastic containers with tight-fitting lids, polyethylene bags and bubble wrap on hand to store the glass as it is recovered.
Store and display glass bottles under the conditions previously recommended for glass objects. In view of the internal stresses present in bottles, control temperature and relative humidity conditions carefully.
Before washing, allow bottles excavated from wet sites to stand for 24 hours in a partially closed bucket with 10 millimetres of water covering the base. This will allow the bottle to adjust to the ambient conditions.
Gradually humidify dried bottles that require washing in a similar fashion.
Remove softened accretions with a nylon brush, such as a toothbrush. Loosen any sand packed inside bottles with a pointed wooden tool and then gently shake it out. Avoid using metal probes as these may scratch and mark the glass. Water will also help loosen and remove packed material from inside bottles.
Soften stubborn accretions and remove soluble salts by immersing the bottles in tap water. Soaking for a few days may be necessary. Do not use warm or hot water as the temperature change may crack the bottles.
Chemicals are generally not recommended for cleaning glass bottles. Although a quick result may be achieved with the use of acids, soda or household bleach, these chemicals will attack weathered glass and leave harmful residues.
An alternative cleaning technique involves the use of an ultrasonic bath. A high frequency sound produces shock waves, which loosen dirt from the glass object. Use this technique cautiously however, as friable glass may disintegrate in the process.
Corks, Caps and Labels
Some bottles may retain the original contents, cork, lead cap or paper label. In order to keep as much information about the bottle as possible, conserve these objects intact.
Paper labels are very fragile and may be obscured by dirt and mould. Remove dirt from the glass and paper by gentle dry brushing. Take precautions so that dust which contains mould spores is not inhaled. A cotton bud moistened with water and wooden points can be used to clean the glass. Do not wet the paper during the cleaning process.
Lead caps on bottles are usually corroded and require chemical treatment to stabilise them. Although details of lead treatment are given elsewhere (see the chapter Metals) consult a conservator before embarking on any treatment of lead caps.
Following all treatments, microcrystalline wax paste can be applied to the lead and any exposed cork to seal the bottle and contents. A suitable wax paste and its method of application are described below.
|microcrystalline wax||5 g|
|white spirit||10 ml|
- apply the paste with a small, stiff brush and allow it to dry; and
- repeat the process until a layer approximately 2 mm thick coats the lead and exposed cork.
If no lead cap is present but the cork is sound and the contents have been retained, the bottle may be cleaned and washed as described in the section above for empty glass bottles.
After cleaning, seal the bottle to ensure the cork remains moist and swollen and that the contents do not evaporate. Apply microcrystalline wax paste to the cork and lip of the bottle (as described above).
Consult a conservator if more information on sealing bottles is needed.
Do not use abrasive powders or chemicals to polish glass. In these processes a layer of the original object is destroyed, breakage may result from heat build-up and chemical residues may be deposited which may cause further damage.