Creating an impression – replicating a medieval seal
It is difficult to appreciate the fine detail of the designs of seals by looking directly at the metal matrices which were used to impress designs into sealing wax. Ideally we would like display and study the positive impression made by the seal matrix but using such historic objects to create an impression risks damaging them. The challenge for the replication specialist is how to achieve a positive impression without risking damage to the original seal matrix.
First, each matrix is assessed for its soundness, and only those suitable are chosen for the work. The selected matrices are cleaned and coated with a lacquer to provide a protective barrier between the object and the mould-making material. The lacquered seal matrix is then impressed into soft plasticene. Room temperature vulcanising (RTV) silicone rubber is then carefully brushed onto the detail of the impression of the matrix. Once this initial layer has cured, more silicone is poured on to achieve the required thickness of the mould. This too is allowed to cure for 12 hours before the finished flexible silicone mould is gently peeled off the surface. This 'first generation mould' now possesses the positive design, as it would appear when impressed into sealing wax.
The next stage is to coat this ‘first generation mould’ with protective lacquer and to apply more silicone rubber in order to take a'second generation mould'. This will have a negative impression of the design, just like the surface of the original seal matrix. This mould is in effect a rubber copy of the original seal matrix and it is from this that the casts will be produced.
Liquid polyester/epoxy resin is pigmented to match the red colour of old wax seals. The mixture is simply brushed or poured into the rubber mould and allowed to harden. When cured the 'cast' can be peeled away from the mould, revealing the perfect impression.