Caring for Your Glass

Wine glass, engraved, twisted enamel threads in stem. George Bacchus & Sons, Birmingham, UK, 1849 - 1851. Museum no. 4449-1901

Wine glass, engraved, twisted enamel threads in stem. George Bacchus & Sons, Birmingham, UK, 1849 - 1851. Museum no. 4449-1901

This section contains Information and advice on how to look after glass. It highlights common problems, tells you what to avoid and provides practical, step-by-step instructions on how to clean and care for glass.

Types of glass

Most glass is a mixture of silica (sand), an alkali (usually soda or potash), an alkaline earth (lime) and a little waste glass (known as cullet). Other materials, typically metal oxides, were added to achieve different working properties or decorative effects, for example lead oxide gave weight and added brilliance, whilst barium produced optical glass.

Glass can be shaped and decorated during manufacture. Once cooled, it can also be embellished by working the surface of the glass, for example by cutting, etching or engraving, or by applying decoration in the form of enamels or gilding.  Glass is often combined with other materials, such as metal (mounts, stained glass), wood (mirrors) or paint and gold (reverse painted glass or verre églomisé). In the right conditions, glass can endure for hundreds of years.

Common problems

The composition of glass can affect its durability: there may be physical impurities in the glass body. Weaknesses can develop during the annealing process in the form of cracks in the glass (annealing is the controlled cooling of the hot glass which allows all parts of the object to cool at an even rate avoiding any stresses).

Avoid dishwashers for historic or valued glass - high temperatures, high pressure water and aggressive detergents can attack the surface of glass to produce an irreversible patchy, cloudy or milky effect.

If your glass has any metal mounts, avoid getting water in crevices where it may cause corrosion. Silver mounts may have been lacquered to prevent tarnishing, so avoid using solvents when cleaning. Mounts may also be attached with plaster or adhesive, which again can be damaged by water or solvents.

Repairing glass objects

Glass goblet - fragments of broken glass. Museum no. CIRC.618-1967

Glass goblet, Pellatt & Co, Falcon Glass Works, England, 1862. Museum no. CIRC.618-1967. Fragments of broken glass bonded with an optically clear adhesive.

Glass is a very unforgiving material - repairs to transparent glass are usually visible, though repairs to opaque, densely coloured or highly decorated glass may be less obtrusive.

Even a shattered glass can be repaired, as shown by this 19th century English goblet. However, damage reduces monetary value of collectable glass, so you will have to balance the cost of conservation against the object's value after repair.

Accidental breakage

If you are likely to make an insurance claim, photograph the scene of the accident before anything is moved. Then collect all the fragments, no matter how small. Fragments can travel quite a distance after impact, so look carefully to find all the pieces. Place larger pieces into a tray or box, padding or loosely wrapping them with clean tissue or acid-free paper.

Use self-sealing bags for small pieces. Avoid attaching sticky labels, which can be difficult to remove. Try not to touch the broken edges, as fingermarks can make rebonding difficult. Resist the urge to try to fit any pieces back together - edges are fragile. Consult a conservator for the best possible repair.

Cleaning techniques

Before you start, remove all personal jewellery such as rings, bracelets and watches. Wearing Nitrile gloves, examine the glass carefully on a padded surface, looking out for the following:

  • repairs
  • signs of crizzling
  • metal mounts
  • gilded, painted or unfired decoration - this will appear fragmentary and damaged and may be lifting or flaking. Unfired gilding rarely survives but if it does it adds historical and monetary value, for example it is occasionally found on heraldic plates. Clean the glass around the decoration but leave vulnerable areas untouched.
  • Flaking or lifting enamel decoration - clean the glass around the enamel but keep swabs well away from the edges of the decoration. Consult a conservator about consolidating the enamel.

If glass is in good condition:

  • use a plastic bowl (avoid using a sink in case you accidentally knock the glass against the taps)
  • use lukewarm water (glass can break if exposed to sudden changes in temperature). If you are in a hard water area, use distilled water rather than tap water
  • add one drop of detergent per litre of water
  • clean one object at a time

Method

  1. Dip the object in the water.
  2. Wipe over the surface with a cotton wool ball to remove dirt. You can use a long handled artist's hogs hair brush for hard to reach areas (wrap the metal band or ferrule with masking tape); a plastic bottle brush can also be useful.
  3. Rinse with clean water either by dipping the glass object into a bowl of lukewarm water (without detergent) or by wiping with a damp swab.
  4. Blot dry using paper towels and allow the inside to dry by carefully supporting the object upside down and leaving it for several hours.

Removing stoppers from decanters

If there are no signs of structural damage (cracks, restored break lines) to the vessel or stopper, one of the following methods can be tried to remove a stopper which has become jammed in place:

  • Slowly heat the neck of the decanter around the stopper with warm (not hot!) water from the tap. Hold the decanter in one hand and gently pull the stopper with the other. Avoid knocking the object against the tap or the side of the sink.
  • Use a small amount of penetrating oil (e.g.WD40, available from hardware shops) - this gradually seeps into the gap and can provide sufficient lubrication for the stopper to be removed.

Removing white deposits from inside a vase

These whitish, cloudy calcium deposits left by hard water can sometimes be removed with a mild acid such as household white vinegar. The vessel should be filled with the vinegar and left overnight, then thoroughly washed, rinsed and allowed to dry.

Glass deterioration

Glass goblet, Venice, 17th century, displaying advance stages of deterioration. Museum no.102-1853

Glass goblet, Venice, 17th century, displaying advance stages of deterioration. Museum no.102-1853

Glass deterioration is usually associated with 16th century Venetian and 17th century Cristallo glass. However, many other types of glass can be of unstable composition, for example, some contemporary glass, glass covers of portrait miniatures or old watches as well as 19th century pressed glass. Glass disease cannot be reversed, but progress is slower at lower temperatures and in drier conditions.

In terms of caring for your antiques, what matters is relative humidity. Relative humidity measures the amount of moisture that is in the air relative to the maximum amount of moisture that the air could hold at that temperature.

'Weeping' glass

17th century glass goblet displaying early signs of instability (‘weeping glass’). Droplets are visible on the surface. Museum no. 2467-1856

17th century glass goblet displaying early signs of instability (‘weeping glass’). Droplets are visible on the surface. Museum no. 2467-1856

If relative humidity is high, affected glass will feel slippery and appear dull. If relative humidity is low, salts will form and the surface will appear patchy or fogged. In subsequent humid or damp conditions, salts on the surface attract moisture and form droplets of liquid. This is known as 'weeping' glass.

It is a good idea to remove the salts as there is some evidence to suggest that leaving salts in place exacerbates the problem. If the surface of the glass is smooth, it can be cleaned with a cotton swab. Lightly dampen (but not wet) this with distilled water then blot the surface dry with a paper towel. This method should not be used when the surface is rough or has begun to flake.

Crizzling

'Crizzling' is the appearance of very fine cracks on the surface of deteriorating glass. The cracks become deeper and more pronounced, the glass begins to flake and eventually the object disintegrates.

 

Produced by the Ceramics and Glass Conservation Studio, V&A - see Conservation


 

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