Desna Greenhow, author of The Diary of Mary Watts: 1887-1904, discusses Mary's notes on 'Cantoni's Lesson in Casting & making Piece Moulds'.

Background:

This 'Recipe Book' was written by Mary intermittently to remind herself over the years of techniques she was using for designing in terra cotta, first for her chapel exterior, and then for her pottery.

Enrico Cantoni 1860-1923, born in Italy, became a teacher in London of techniques for sculptors in moulding plaster casts for pieces of sculpture. He was also a bronze founder. He worked at the Central School of Arts and Crafts giving demonstrations “for the benefit of modelling students", and also taught at the RCA. Cantoni worked with the sculptor Conrad Dressler, who Mary knew well, and also made plaster moulds for Aimee-Jules Dalou, who taught Mary to work with terra cotta when she was a student at the Slade.

'Recipe Book':

Necessities.

John Knight's soft Primrose Soap
Some modelling clay
Plaster. There are 3 kinds – Superfine for modelling – Finefor moulding – Coarse for Builders
Cafferata at Newark. A good firm
Oil. Rape oil preferable
A sharp knife
Spatula & brushes
A stone for sharpening your knife

The soft soap must be prepared thus – 1\2 lb of John Knight's soft Primrose soap to 1 quart water. Boil all together. In consistency the soap should run almost like water. If too thick when boiled add more water.
Take a lump of modelling clay. Knead it well & make a bed of it with care & well smoothed out. Into this the figure that has to be cut must be pressed, taking care that something less than half lies buried so that the lower half of the mould comes away quite easily & can be made in two pieces, sometimes in one.
The clay bed must be very carefully levelled & smoothed on the surface, as this represents the main point of the mould.
Soap the terra cotta figure well with a lather then smooth & oil with Rape oil.
Mix the plaster, flouring it into the water very carefully & finely. Do not throw in great spoonfuls at a time. The plaster should be on the surface of the water & should be a stiff mixture which should set hard in a quarter of an hour. It begins to set in four minutes time. Apply the first coat of plaster to the left side of the figure, first painting it on very carefully, & avoid air holes. For this use a medium-sized brush, & then build up with the spatula, half an inch thick or more according to the size of the terra cotta figure.

Number 1 hardened in 4 or 5 minutes during which time Cantoni had made Number two. He then trimmed up this with the sharp knife. The knife was sharpened each time on the stone. Each time, as he added 2. 3. 4 etc. he painted the plaster with Rape oil. Before moulding the parts of the terra cotta where there was undercutting, Cantoni filled these up with clay. This is very important as the mould must fall away very freely. On each part of the mould he put a rough knob of plaster temporarily there for tapping with the knife handle to loosen the different parts.