Broken glass in a longcase door leaves no option but replacement.
The restorer’s task is to replace it sympathetically.
First, there is the glass itself. Modern float glass is too flat and looks wrong in an antique door. Horticultural glass is too heavy. The answer is to use picture glass from old prints and photographs available cheaply from auction. Glass should be approx. 2mm. thick. Old glass is often brittle, and cutting it into odd shapes is a job for an expert glass cutter.
Removing the old glass
Earlier cases have a fairly robust rebate in the door frame and the old putty can often be chipped out with a chisel, but many Victorian clocks may have only a layer of veneer which is easily split. Heating the putty with a soldering iron or paint-stripping heat gun softens it for long enough to remove it safely.
Fitting new glass
Most old makers did not fix the glass with nails, and it is not necessary. With the door flat on the table, lay a thin bead of putty in the rebate and press the glass onto it. Work putty into the rebate with the thumb, and level off with a putty knife.
This brings us to the real purpose of this article – the putty. Linseed oil putty sets too slowly; plaster of Paris is too brittle and quick-setting. There are modern types of mastic that will do the job, but I wanted to find a recipe that could have been used when the clock was made.
I once read that old clockmakers used flour in making putty for fixing the glass. Although I can no longer locate the reference, it served as the starting point for this investigation.
Permanent. The glass should stay in place for the life of the clock.
Flexible. The putty must be flexible enough to accommodate movement of the wood frame as it absorbs or loses moisture.
Workable for a reasonable time.
Authentic. The main point of this enquiry was to discover a recipe as close as possible to that used 200–300 years ago.
Putty hardens with age, suggesting that a hardening oil such as linseed is part of its makeup.
Heating the material is accompanied by a smell of linseed oil.
I tested pieces of old putty with iodine. Every piece of genuinely old putty gave a blue/black colour, confirming that a starch-based material was used as a binder.
Crumbs of the same material were tested with acid. A fizzing was observed in each case, confirming the presence of a carbonate (i.e. calcium carbonate/chalk).
This gives us three ingredients: linseed oil, starch (flour) and chalk. An analogous and well-documented material is gesso, as used for gilding.
Gesso is made from whiting (ground chalk) and animal skin size or glue. Clearly we need some kind of glue, but there is no evidence of animal skin glue here, so flour paste would seem to be the obvious choice.
Mix ½ tablespoon of flour to a paste with a small amount of water.
Add ½ pint water a little at a time, stirring to avoid lumps.
Bring the mixture to the boil and simmer until it thickens and clears, stirring frequently.
The resulting paste can be stored in the freezer.
- Put a tablespoon of paste in a bowl and stir whiting into it.
- Keep adding the whiting and kneading until the stickiness disappears and it is fairly dry.
- Knead in a small quantity of linseed oil until the putty becomes smooth and flexible.
This works well. It sets hard overnight, but slowly enough to make it easy to use. There is slight shrinkage, but any small cracks that may appear on drying can be filled easily with wet putty. This formulation is perhaps not as flexible as I would like, and mixing is an annoyingly sticky job.
To improve flexibility and reduce stickiness, I wanted to increase the oil content, and to add it at an earlier stage, so I turned to a well known culinary technique. Anyone who has made white sauce will be familiar with making a roux.
Heat a teaspoonful of linseed oil in a saucepan.
Add half a tablespoon of plain flour.
Cook this mixture, stirring constantly, for 2 minutes. This heating is essential to burst the starch grains.
Add ½ pint water, a little at a time, stirring continuously. Adding the water slowly avoids lumps.
Simmer for a few more minutes.
The result is a quite appetising-looking yellow custard, although I wouldn’t recommend it for consumption. Leave it to cool. It should be frozen if you cannot use it all in a day or two.
The rest of the process is the same as before, but a lot cleaner.
Put 1 tablespoon of ‘custard’ into a bowl.
When it is thick enough, knead the mass by hand and keep adding chalk until a firm, but flexible putty is produced.
The mixing is much easier than before and this material is exactly right for the job. It dries a little more slowly than the original formula, but sets firmly overnight. It will keep well in the freezer. The proportions used are approximate and experimentation may be needed to arrive at the perfect formula.
Bright white putty looks too new on an old clock. I tried mixing colour at different stages of the putty-making process, but it takes a lot of pigment to produce any depth of colour, and affects the consistency of the final product. I have no doubt that more experimentation with incorporated pigment could achieve good results, but I rejected it in favour of surface painting. Both water- and oil-based paints take well on the putty surface (as do acrylics, but we are looking for authenticity).
If you want to use materials that any clockmaker would have in the workshop, a mixture of rouge and rottenstone in flour paste or glue gives an excellent reddish-brown finish.
Few clock owners are likely to worry about the composition of putty and I wondered if I was wasting my time, but I was surprised and delighted by the superior quality of the finished product. It is much easier to use than the modern equivalents, it costs almost nothing and customers cannot fail to be impressed by their clockmaker’s attention to detailed authenticity.
I purchased whiting from The Gilders Warehouse Ltd., Units 5 & 4D, Woodside Commercial Estate, Thornwood, Epping, Essex CM16 6LJ. www.gilders-warehouse.co.uk.