Making a Cornice Running Mould: The Frenchman and Horse

A running mould is one of the most reliable ways to reproduce a clean, consistent cornice profile in plaster. The method uses a shaped former, known in many workshops as the Frenchman, mounted on a travelling support called the horse. As the tool moves along a straight guide, it cuts or compacts the plaster into the required decorative line.

For Australian plasterers, the technique is useful on new interiors, Federation renovations, Victorian terraces and heritage repair work. It suits long lengths of cornice far better than trying to shape every section by hand, provided the profile is accurately drawn, the guides are stable and the plaster is kept at the right working consistency.

Understanding The Frenchman And Horse

The Frenchman is the shaped profile board or template that carries the negative form of the cornice. Its cutting edge must reproduce the finished section accurately, including beads, coves, fillets and any small quirk lines. It is commonly made from zinc, mild steel, aluminium or a hard-wearing board laminated with sheet metal. The choice depends on how many metres are required and how abrasive the mix will be.

The horse is the travelling frame that holds the Frenchman square to the work. It may be a timber carriage, a compact metal runner or a purpose-built jig with adjustable stops. The name varies between plastering shops, so some tradespeople use “horse” for the whole sliding carrier and others use it for the support rails. Before building, confirm the local terminology and draw the complete assembly so the former, carriage and guide battens work as one system.

The tool needs enough bearing surface to remain steady without becoming awkward to push. A narrow Frenchman can chatter, while an oversized horse becomes difficult to control at corners and returns. For a long Sydney hallway or a heritage ceiling in Melbourne, a balanced tool with a comfortable handle will usually produce a better result than a heavy, overbuilt frame.

Drawing And Building The Profile

Begin with a full-size section of the proposed cornice. Draw the wall line, ceiling line, shadow gaps and finished arrises on a sheet of plywood or rigid board. Check the dimensions against the room, because a profile that looks elegant on a small sample can appear oversized in a low Queensland ceiling. If matching an existing moulding, take several measurements rather than relying on one damaged or uneven section.

Transfer the profile to the Frenchman with care. The edge that contacts the wall and ceiling must be square and true, while the plaster-forming edge should be clean enough to leave sharp details. Add a small clearance where the tool needs to pass over a guide or bedding coat. Mark the face of the former clearly so it cannot be fitted backwards when the work becomes busy.

Build the horse around the actual Frenchman, not the other way around. The carriage should hold the former rigidly and keep it perpendicular to the running direction. Fit replaceable wear strips or low-friction runners, then test the movement over a dry length of guide. Any rocking, binding or side play will be transferred directly into the cornice as waves, steps or inconsistent shadows.

Preparing Guides And Plaster

Fix straight guide battens to the wall and ceiling, allowing for the required projection of the cornice. Sound timber, straight aluminium angle or carefully set plaster grounds can be used. The guide must be secure enough to resist the pressure of the horse; relying on a few loose screws or soft substrate will cause movement. On older Australian cottages, check for lath, uneven masonry and previous patching before choosing fixings.

Set the guides with a level, straightedge and profile gauge. A laser level is useful across long rooms, although it still needs to be checked against the actual substrate. Leave enough room for the bedding coat and finishing passes. Where a ceiling has noticeable sag, decide whether the cornice should follow the existing line or establish a corrected datum. That decision is especially important in heritage work, where a perfectly level line may look wrong beside an old ceiling.

Mix a plaster suitable for the build-up and working time. A firm backing coat can support the bulk of the section, followed by a finer finishing material for the face. The finish should be workable without slumping, but not so dry that the Frenchman tears the surface. NAPL’s guide comparing Thistle finishing materials is useful when selecting a compatible finish for the substrate and the required surface quality.

Running The Cornice

Apply the backing plaster generously between the guides, leaving enough material for the Frenchman to engage fully. Position the horse at the starting point and draw it steadily along the run. Keep even pressure against both guides and avoid stopping halfway through a pass. Several light passes are safer than trying to remove all excess material in one push, particularly with a deep cove or enriched decorative profile.

Clean the former after each pass. Dried crumbs on the edge can create repeating ridges, while a wet build-up can soften small details. Use a damp brush, scraper or sponge without rounding the profile. At internal corners, run the main length slightly past the junction, then cut and mitre the return once the plaster has firmed enough to hold a crisp edge.

External corners and short returns need planning before the first mix is made. Pre-run small pieces on a board where possible, or create a stop-end that allows the horse to finish cleanly. For repairs in a narrow terrace or apartment, make sure there is enough room to lift and reverse the frame without damaging the ceiling, cornice or freshly painted walls.

Refining, Joining And Protecting The Work

When the plaster has tightened, refine the face with a fine tool and remove slight drag marks. Do not overwork the surface while it is still soft, as excessive brushing can pull the corners out of alignment. Once dry, sand only where necessary. A properly made Frenchman should leave the profile close to finished, reducing the need for aggressive rubbing that can flatten beads and alter the original character.

Long runs are joined with a neat scarf or mitred joint. Cut both ends cleanly, dampen the contact surfaces and use fresh plaster to bed the pieces together. Keep the joint away from a prominent feature where possible. For a decorative fibrous cornice, scrim or suitable reinforcement may help stabilise a difficult join, particularly where the substrate moves between masonry and plasterboard.

Inspect the surrounding building before committing to a new run. Cracking, dampness, movement or failed fixings can spoil an otherwise accurate moulding. If work is near external walls or a suspected underground leak, review this water-main warning before disturbing the area; ground movement and moisture can affect both render and interior cornice lines. In coastal areas such as Newcastle or Adelaide, salt exposure and humidity also make sound preparation and suitable primers important.

A good running mould is the result of measured preparation rather than force. Store the Frenchman flat, clean the horse after use and record the profile dimensions for future matching work. Apprentices can learn the method quickly by making a small practice length first, checking the section with a profile gauge, then progressing to a full room.

Build your next cornice project around accurate drawings, a rigid horse and a carefully finished Frenchman. For plastering resources, workshop guidance, trade updates and training information suited to professional practice, join the NAPL email list and keep the method close at hand when the next restoration or decorative finish arrives.