Agree the methodology
A one-page method statement covering in-situ moulding, split positions on the fillets, and trial casts approved on site.
Project record · Heritage window tracery
Reproducing Gothic window tracery for a parish church.
Two lights of a Victorian Gothic window had lost their tracery to frost. One matching light survived. A split silicone mould from the survivor gave the mason's yard a clean route to matched cast-stone replacements.
The brief
The brief in one paragraph.
A heritage stonemasonry contractor was working on a listed parish church where frost had broken up the tracery in two lights of a three-light window. The third light survived with light surface loss. The conservation team were open to cast stone for the replacements provided they matched the survivor in profile and surface, and the contractor asked us to make the tooling.
The problem
Tracery is all undercut.
Tracery is pierced on both faces with deep cusps and foils. A one-piece mould couldn't release it, and a crude split would leave witness lines across the very profiles the conservation officer would look at first.
The surviving section also had to stay in the window. Removal was ruled out, so the mould had to be taken in situ, from a scaffold, on a cold elevation.
How we solved it
How we approached it.
In-situ silicone moulding
Brush-applied silicone in layers over a barrier, with a GRP mother mould built in sections so it could be lifted off the tracery safely.
Bench pattern
A plaster positive cast from the in-situ mould, lightly made good where surface loss was agreed to be restored.
Production split mould
A multi-part poured silicone mould from the pattern, with splits on the fillets and matched register keys.
Trial and approve
Two trial casts in different stone-matched blends taken to site and held in the window opening.
The outcome
How it worked out.
The contractor cast the replacement tracery in the approved blend and installed both lights. The split lines fell on the fillets as planned and were dressed out by the mason in minutes. The production mould is archived against the church in case the survivor itself needs replacing in future.
Lessons
What we'd tell the next person with this problem.
On pierced work, spend the time deciding split positions on the pattern with the mason in the room. The person who'll dress the casting knows exactly which edges are easy to clean up and which aren't.
At a glance
The idea in a picture.
How a two-part mould fits together
Swipe to see the whole diagram
Get in touch
Send us a photograph of the panel or the elevation.
One photograph tells us what surface you're chasing, and one line about the mix tells us what silicone or polyurethane will survive it.
Real people, a real workshop
You deal with the people who make it.
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