Clock glass
Clock glass can be divided into three distinct categories, regardless of its shape. The most important characteristic is the way in which it is held in place.
Rough-edged glass
Common on American clocks and English & German mass-produced clocks, rough-edged glass is normally held in place by some ring or fitting. The edge of the glass is hidden by the bezel, and although the retaining system can sometimes be very difficult to dismantle, replacement can be done in the workshop.
When the retaining ring has been removed, measure the size of glass needed. Modern glass is 2mm thick, but some clocks used 1.5 or even 1mm. If this is the case, you will need to thin down the edge with wet abrasive paper. Flat glass is normally available from a local hardware shop.
During the manufacture of convex glass, a flat circle is placed over a concave tin mould and placed in a kiln. The glass reaches the point at which it will just sag into the tin mould, and the kiln cuts off. It has to cool down slowly, to anneal the glass. The slightest draught of cold air will harm the glass and it will probably crack sooner or later.
The problem with this method of manufacture is that the glass reduces in diameter when it sags, so the manufacturer has to place oversize glass on the tin mould before firing. This makes the manufacture of accurately sized convex glass quite difficult. You can normally expect rough edged glass to be within 1mm of the size stated, and most clocks using rough edged glass will accept glass within this range. Mass-produced items like bottles and table glassware are made by other methods and the dimensions can be controlled.
Another method of retaining rough edged glass is with glazing plaster. This is a tougher version of the Plaster of Paris used to protect broken limbs.
When re-fitting glass with glazing plaster, a good trick is to use some glue such as a hot melt gun or Epoxy resin to hold the glass in place while you work. A small spot in three places will suffice. A skilled glazier wouldn't bother, but you will find that fresh plaster sets quite quickly and you won't have enough hands.
Stale plaster will set, but its hardness will reduce to a chalky powder. You can control the speed at which plaster sets by adding a little table salt.
Snap-in glass
The majority of French clocks and cast English clock bezels have a groove into which the edge of the glass fits tightly. Re-glazing these is a highly skilled job. Glass has to be ground down to within 0.25mm and then the bezel is heated in hot water to expand it. The glass is then finally adjusted so it just 'snaps in' and when the bezel cools, the glass is held tightly.
Although a bezel may look fine, it can be easily distorted, especially if the clock has been without a glass for many years. Not only must it be truly round, it needs to be flat or the glass will want to pop back out. The glazier may have to repeatedly adjust the glass before a good fit is obtained. Some bezels are simply too out-of-shape to glaze properly. Always check that the bezel groove is flat (in one plane) before sending it away to be glazed.
If the glass is too tight, it will crack on a cold day, when the shrunken bezel puts excessive pressure on it. It is not unknown for a repairer to put a valuable clock in a cold strong room in the basement overnight and find it with a cracked glass in the morning!
Bevelled glass
Carriage and Bracket clocks often have bevelled glass. This is not as critical as the snap-in variety; however cases are often not as square as they look. If you want the glass to be an exact fit and not rattle, have the glass made to fit the clock case and not to just to a paper template or written dimensions.
It is quite common to see small pieces of paper or gum concealed between the edge of the glass and the carriage clock case, to take up slack.
Local glaziers often undertake this kind of bevelling now that industrial abrasives have become more easily available, although they may be reluctant to get involved with the clock case. You can get them to make a very slightly oversized piece, and rub the edge on coarse wet abrasive paper or a diamond coated file until it fits.
This document is © copyright M&P 2005
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