Composition ornament ("Compo") is a mouldable resin worked either by hand or more usually pressed into moulds to produce decorative work. It's now most commonly seen as part of gilded picture frames, but was in use for many smaller decorative mouldings from the later part of the Baroque period.
A Compo recipe
A basic compo recipe, which can have many variations.
Take 250 grammes cabinetmakers' pearl glue and soak in adequate cold water for several hours. Drain the excess water, then heat in a double boiler or electric gluepot to 70°C until liquid. Add 20 mls of glycerine and 10 grammes of zinc oxide.
Powder 100gms of rosin (colophony) and heat in a small pan. Beware of fire! The rosin should melt at around 90°C. Add 20 mls Venice turpentine and 60mls linseed oil, and mix well. Mix the melted rosin etc. into the hot glue and stir well.
The compo itself can now be made by pouring the combined contents above onto 600-700 grammes of whiting in a bowl. Stir the initial hot mixture with a wooden spoon.
Rub talcum powder on your hands (to prevent sticking), then work the mixture by hand until it has the consistency of glazing putty.
Work the compo into conveniently-sized flat cakes, then wrap them well in kitchen cling-film (Saran wrap). Compo will keep in a freezer for a month. Defrost the compo completely before placing it in a bain marie to soften it ready for moulding.
The recipe proportions aren't rigid. The pearl glue can be increased to 300 grammes for a harder and stronger result, or then the rosin to 150 grammes for a more elastic and generally more robust composition. Some workers prefer to use rabbit-skin glue (as used in bookbinding) rather than pearl or hide glue, giving a more flexible result.
Some understanding of what each ingredient is doing helps when adjusting the basic recipe:
- Whiting gives body.
- Pearl glue acts as a binder.
- Linseed oil makes the mixture soft.
- Rosin makes the mixture elastic.
- Venice turpentine prevents cracking.
- Glycerine tempers the glue.
- Zinc oxide prevents formation of mould.
Composition is widely accepted to have been developed by Thomas Jackson in London around the late 18th Century. His son, George Jackson, continued the business. His company, George Jackson, still trades today and supplies composition ornament made to the original methods and recipe.