Compo is a moldable substance worked either by hand or more usually pressed into moulds to produce decorative work. It is most commonly used as part of gilded picture frames, but was in use for many smaller decorative moldings from the later part of the Baroque period.
There are many different compo recipes with variations in ingredients and proportions. However, all compos should have ingredents that give it certain qualities. Understanding the function of each ingredient helps when adjusting the basic recipe:
Whiting (gives body)
Pearl bone glue (acts as a binder)
Linseed oil (makes the mixture soft)
Rosin (makes the mixture elastic)
Venice turpentine (plasticizer to prevent cracking)
Glycerine (prevents the glue form drying and acts as a plasticizer)
Zinc oxide (prevents formation of mold)
Soak 250 grams pearl bone glue in sufficient cold water to immerse the glue for several hours. Drain the excess water, then heat in a double boiler or electric gluepot to under 70° C until liquid. Add 20 mL of glycerin and 10 grams of zinc oxide to the liquid glue and stir well.
Crush 100 grams of rosin (colophony) flakes to a powder and heat in a small pan. Rosin melts at around 90° C. Do not overheat as it may catch on fire. Add 20 mL Venice turpentine and 60 mL refined linseed oil to the melted rosin with stirring to mix well. Mix the melted rosin, oil and Venice turpenine into the hot glue and stir well.
Make the compo by pouring the combined contents above onto 600–700 grams of whiting in a bowl. Stir the initial hot mixture with a wooden spoon.
Rub talcum powder on your hands (to prevent sticking), and then work the mixture by hand until it has the consistency of putty.
Work the compo into conveniently-sized flat cakes, then wrap them well in plastic 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 molding.
The proportions and ingredients in this recipe aren’t rigid. The pearl glue can be increased to 300 grams for a harder and stronger compo, or the rosin to 150 grams for a more elastic and generally more robust composition. Some workers prefer to use rabbit skin glue rather than bone or hide glue to give the compo a more flexible product.