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Composition or 'compo': an alternative to plaster and woodcarving

Usually confused with plaster, composition or ‘compo’ has unique qualities and history. While plaster is formed by mixing sand, gypsum or lime with water, compo is the result of mixing glue, oil and resin with a filler, usually chalk. A traditional compo recipe would contain glue produced with animal tissues, linseed oil and the once cheaper pine or pitch resins. The exact formula of composition varies depending on the supplier. Its main characteristic was its sharp definition of details, its elasticity at the time of installation and its capability for repetition with the craft skill reduced to the carving of a single mould.

Detail cornice of compo versus plaster

The origins of composition

An unfamiliar material today, it was in high demand during the 18th century. The British fashion for Ancient Roman detailing made of compo the go-to material for reproducing such ornament. Precedents for moulded decorations existed in Medieval and Renaissance times, but it was in 1780 that compo was invented in England. The creation is attributed to George Jackson, whose company is still active and producing compo ornaments and plaster ornaments. One of its more famous customers, the eighteenth-century architect Robert Adam, was a crucial figure in the promotion of compo. Its popularity was such that it was sold by catalogue. In the following centuries, Ruskin’s demand for honesty in the use of materials and the absence of detail promoted by modern architecture brought composition into oblivion

Once prepared the compo dough is left to rest

Why composition rather than plaster?

Composition was widely used for internal ornaments, even though a few examples remain of its use for exteriors using white lead instead of glue. Compo was applied as an alternative to woodcarving, stone carving, and plaster detailing. Some of its most common applications are cornices, architraves, wall panels, picture rails and dados, and furniture decoration. In contrast with wood and stone carving, the craftsmanship required is lower. And at the same time, compo has a greater capacity to replicate details. This brought the cost down while the quality of the pieces was higher, sharper than that resulting from plaster ornament. It was the ideal material for commercial, residential, and speculative developments.

Hardwood mould to create composition pieces

Making compo

The preparation of composition starts with the mixture of its ingredients to form a dough. This is left to rest until ready to be put into a negative mould. The case mould is then pressed to expand the dough into the intricate edges of the internal carving. Once removed from the mould, it is trimmed and ready to be fixed. A compo ornament can be softened with steam to increase flexibility during the installation. Steam also activates the glues in it; this improves its adhesion to the substrate or the end edges of different pieces if part of a run. Wire and pins could also be used for fixing and improving the stability of bigger pieces. Once installed, it is ready for decoration.

Georgian hardwood mould

Compo performance and care versus plaster

Compo’s elastic nature allows its substrate’s expansion and compression without cracking the ornament. Nevertheless, its resilience can vary depending on the original recipe and the climate conditions suffered by it and its substrate. Maintenance and repair might be occasionally required, but its greater risk often comes from being confused with plaster. As a water-soluble material, the cleaning of compo differs from that of plaster. For instance, a caustic paint stripper suitable for plaster would disintegrate compo. Its light or darker brown colour can help with its visual identification. Still, its chemical analysis would be required to better understand its formula.

Press to expand composition into corners

Repairing compo

If compo ornament fails, the cause and the state of damage should be identified and assessed. Superficial cracks might be acceptable, but deeper ones might require repair, replacement, or refixing of fallen elements. No repairs should be carried out until the source of the problem is solved. If missing features are reinstated, listed building consent might be required. You might be exempt from consent when doing like-for-like repairs and maintenance work. Nevertheless, you should discuss this with the local conservation officer. Regardless, a historic and architectural study of the building and the ornament at stake will warranty the correct repair strategy, including selecting a suitable material, design and maintenance.

If you want to discuss a particular case, feel free to contact Historic Building Studio.

George Jackson samples from their library
All images were taken at George Jackson's workshop

This article was first published in the Nov/Dec issue of 2022 of Listed Heritage, the magazine for The Listed Property Owners’ Club magazine.