Glass is a random amorphous material made by fusing silica (SiO2) with an alkali, usually either sodium oxide or potassium oxide, and lime. The most common glass type is soda-lime glass, but there are other types of glass, including lead glass, which contains at least 20% lead oxide. Glass may be colored by the addition of metal ions (cobalt, copper, manganese and iron).
Factors that can accelerate decay in glass include: the type of glass and the manner in which it was manufactured, the amount of moisture, alkalinity, and salts in the burial environment and physical wear or abrasion prior to burial.
Water is the main cause of environmental decay. As water reacts with the glass, causes ions to leach out opening up the glass network and eventually leading to dissolution of the glass structure. Highly acidic or highly alkaline environments will also cause glass to decay.
The most common forms of visible decay seen in glass are listed below:
Iridescence: Glass decays from the outer layers inward, forming thin, onion-like layers that appear opaque and/or different in color from the original glass because the way in which the light is refracted through the glass has been altered.
Exfoliation/Spalling: As the decayed layers separate, they flake off, causing the original surface of the object to be lost.
Devitrification: Deterioration resulting in a crystalline and iridescent appearance.
Pitting: Small isolated holes or indentations in the glass surface or on broken edges.
Surface Abrasions: Damage to the glass surface as a result of use or burial.
Cracking: Cracks running throughout the glass structure.
Although most glass can be lifted without additional support, during excavation, treat all glass as though it is fragile. Remove as much loose dirt from around the artifact as possible before lifting it. If there is dirt sticking to the glass surface, do not try to remove it in the field, as it may be helping to hold fragile layers of the glass together.
If glass artifacts are found dry, pack them inside sealed polyethylene bags in well-padded containers for transport to the lab. If glass is found wet, it must not be allowed to dry out or else the surface layers will begin to shrink and delaminate. Do not attempt to remove dirt on the surface of wet glass. Store wet glass in sealed polyethylene bags and transport the glass as soon as possible to a lab. An alternative solution is to store wet glass in punctured, sealed polyethylene bags in plastic bins containing damp polyethylene foam. Store these containers of glass in a cool place until they can be properly cleaned in a lab. Glass should not be stored wet for a long period of time.
If the glass is dry and in good condition, dirt can be removed with a soft, dry brush and soft wooden picks. Dirt removal is easiest while the soil is still damp. If the glass is very decayed, iridescent, or exfoliating, do not attempt to remove the dirt, as the outer layers of glass may flake off during cleaning. The opaque layers of glass may contain decorative elements, such as hand painting and etching. They also preserve the original size and shape of the glass piece. Removing these outer layers of glass, irrevocably alters the surface of the piece and removes important information.
Iridescent, weathered glass, either wet or dry, may need to be consolidated to prevent the opaque layers from exfoliating further. Consolidation should be left to a conservator, or undertaken only after consultation with a conservator. Consolidation reinforces the layers by adding a material between the layers to replace lost material. It will aid in keeping the opaque layers intact, without altering their appearance, morphology or history. Acryloid B-72 is often used as a consolidant for archaeological glass. This particular resin is very clear and remains so after treatment, so it does not interfere with the appearance of the glass. It is also hard upon drying, without being too brittle, and has been shown to be most like a glass object compared to other adhesives.
Consolidation can occur in one of two ways: by surface application or by immersion. Techniques for surface consolidation include brushing the consolidant onto the surface and injecting or dripping it into the artifact. It is a useful technique for very fragile artifacts, those with loose paint or enamel, and large objects; however, the drawback is that the consolidant may not fully penetrate the artifact, consolidating only the surface layers. This results in a strengthened exterior and a weak core. As the artifact is handled, the exterior may separate from the interior. Consolidation by immersion involves immersing the artifact in the consolidant, if the artifact is not too fragile to withstand it. In order to avoid tide lines the artifact must be fully covered. Sometimes this technique is carried out under a vacuum to ensure that the consolidant is drawn fully into the artifact. Consolidation by immersion requires larger amounts of consolidant than surface applications and can pose health and safety hazards depending on the consolidant used. Vacuum impregnation should only be carried out on robust artifacts as the vacuum can damage very fragile pieces.
Proper cleaning, documentation and drying of the glass pieces must be performed in coordination with any consolidation efforts. Care must be taken when drying the artifact to avoid surface pooling of the consolidant.
The consolidation of waterlogged glass is complicated by the need to replace the water absorbed by the glass with a suitable solvent system. Only a conservator should undertake it.
Once clean and dry, glass artifacts should be kept in an environment free of fluctuations in RH and temperature, and free from dust and vibrations. Glass must not be packed too tightly, but should be sufficiently padded in storage with either Ethafoam or with acid-free tissue. Handling also poses a risk to glass, and all fragments and artifacts should be handled with clean hands over a padded surface to avoid damage.
Newton, R. and S. Davison. (1989) Conservation of Glass. Oxford: Butterworth-Heinemann.
What adhesive do you use to mend glass?
The most important factor in choosing any adhesive to use on archaeological artifacts is the long-term stability of the adhesive. Adhesives that discolor, become brittle, fall apart, or cannot easily be reversed (re-dissolved in order to undo the join) are not appropriate for archaeological materials. Many adhesives readily available from hardware stores, craft stores and other suppliers may be appropriate for fixing a broken coffee mug or other household item where preservation is not a factor however they are not appropriate for use in museums or other cultural institutions. Many such adhesives have been used on archaeological glass in the past and have caused more harm than good.
Adhesives for all archaeological artifacts should be conservation-grade and should be purchased from conservation suppliers for the following reasons:
The adhesives sold by conservation suppliers tend to be purer mixtures than those sold elsewhere. They contain only the resin and solvent whereas many commercial adhesives include plasticizers and fillers that may not be archivally stable over the long term.
Most commercial manufacturers use proprietary formulas to make up adhesives. These formulas may change from batch to batch with no warning so that an adhesive that may once have been a suitable material for preservation use is no longer suitable. Unless you have the resources to test each batch for suitability prior to use, it is better to avoid commercially manufactured products altogether.
Another factor in choosing an adhesive for glass is that the adhesive should not be stronger than the glass itself. In the case of physical shock to the object or mechanical failure of the join, it is better for the adhesive to fail at the old join than for the glass to break in new place because the mend is so strong.
Adhesives traditionally used to mend archaeological glass such as cellulose nitrate (Duco Cement, HMG Blue Tube) and cyanoacrylate (Superglue) tend to be stronger than most glass structures. They also tend to be irreversible and do not age well, yellowing over time and becoming brittle and insoluble as they begin to cross-link from age. Most proprietary adhesives are intended for temporary use, and therefore are not suitable for conservation purposes.
Acryloid B-72 is often suitable for archaeological glass, since it does not change color, and is easily reversed with solvent, if that becomes necessary. There are some instances, however, when it is not the best choice for glass. If the glass is in excellent condition, there may be no pores to help the glass piece bond with the B-72. In these cases, stable conservation-grade epoxies, such as Hxtal NYL-1, can be used. The working properties epoxies vary, and must be used properly in order to get a good bond. Hxtal, for instance, takes 7 days to fully set.
What do you use to fill gaps in glass?
Any material used for filling gaps in glass must have the same properties as an appropriate adhesive for glass. See What Adhesive do you use to mend glass? A glass fill, however, also needs to be transparent or translucent. The more transparent the artifact, the more difficult it is to create an acceptable fill. The condition of the glass has an effect on the choice of gap filling material as well. For example, for small holes or line-chips on a mended artifact where surface deterioration has rendered the glass translucent, Acryloid B-72 to which an inert fill material has been added, such as fumed-silica or glass balloons may be used. However, this may not be suitable in all cases. Acryloid B-72 shrinks slightly when it cures, so it is impossible to use it as a clear casting material for larger missing areas.
Most clear synthetic resins that are designed as casting materials and are available to the consumer will yellow with prolonged exposure to light and are difficult to reverse. Hxtyl-NYL is a conservation grade epoxy (the hardener has been purified more thoroughly than most epoxies, so it’s light fastness is much better than most epoxies, but it still not as light-fast as Acryloid B-72). For mended glass artifacts that are in very good condition (dense and clear), it is possible to create an acceptable fill with Hxtyl-NYL Hxtyl-NYL requires special handling and safety precautions for both its use and reversal, as well as excellent manual dexterity and considerable practice in using it for this purpose.
The scholarly examination of archaeological glass and many exhibit situations may necessitate mending the artifact, but rarely requires the filling of larger gaps. If however, for aesthetic reasons or display purposes, a glass artifact requires fills, it is highly recommended that a conservator with experience in this area be contracted to perform this kind of work. Creating appropriate and effective fills for glass of any kind requires significant experience with the materials in order to be successful. Experimentation is not encouraged, as unsuccessful fill attempts subject the artifact to additional unnecessary reversal procedures and can create damaging stresses within the artifact.
Sourche from; http://www.sha.org/research_resources/conservation_faqs/treatment.cfm