Most frames, we regard as “old” were crafted by hand.
Ornaments were made of a plaster like substance called “Compo” and then adhered onto a wooden profile.
After drying they were finished off with a water or oil gilding technique. Compo is a mixture of chalks and animal glue mixed and kneaded together until it reaches a dow like consistency
Over time, through changes in humidity and temperature, and the fact that wood moves/shrinks, these ornaments can crack.
In severe cases they loose contact with its wooden support, and fall off.
Different stages of frame restoration:
Replacements are done by making a mould of an existing ornament, casting and fitting it.
The material for replacements is mostly plaster of Paris.
In some cases one has to resort to more modern materials, like resin.
If there are no existing ornaments remaining, one has to re-model the item entirely.
COMPO, GESSO, KREIDEGRUND
… all kind of the same … made of chalk, glue and oil, used as a primer or a modelling material .
Gesso is made of a variety of chalks that are mixed together with animal glue in a certain ratio.
This is a wet and warm process and involves a quite a bit of water.
Recipes vary widely, depending on what kind and quality of chalks and glues are available.
The viscosity of the Gesso can range from a liquid to a dough like substance. The type and ratio of chalks and animal glues used determines how well the Gesso flows or not.
For frame ornaments the “Kreidegrund” needs to have a dow like consistency. This would be referred to as “Compo”.
Used as a primer for canvas (paintings) or wood (sculpture) it would have to be more runny, like condensed milk. This is called “Gesso”
Over the years a gilder develops a fine feel for what is required to make the perfect Gesso for whatever individual use and purpose.