A plasterers cornice run in-situ was a traditional way of forming a decorative moulding, onsite, in a room at its ceiling to wall junction. Many Victorian properties will have in their rooms cornice of this type.
A cornice run in-situ on the ceiling/wall could be formed in lime sand mortar, lime putty, or gauged setting stuff.
Usually a cornice was cored out in a lime sand mortar using a running mould with a muffle on it. After a setting period had elapsed the running mould, which would have been cleaned and had its muffle removed, was then used to finish the cornice in gauged setting stuff.
Names of Mixes
Lime Sand Mortar: Lime putty is used as a binding matrix and is mixed with plastering sand used as an aggregate. Usually the mix was in proportions of 3 or 4 sand to 1 lime to form a workable mortar with the addition of a small amount of water.
Lime Putty: This is quick lime (also known as lump lime or burnt lime) which is slaked with water, often in a lime pit, to form a lime putty. Lime putty produced fatty, lively workable mortars which set slowly by carbonation.
Gauge Setting Stuff: This is a mixture of sand, lime putty and gypsum. A very fine sand is used in gauge setting stuff with the volume of sand greatly reduced in the mix, the lime putty and sand usually batched in equal proportions. Gypsum speeds up the set and produces a smooth, hard finish.
A plasterers running mould is made up of a stock (piece of timber with a metal profile sheet fixed to it). A slipper is a piece of timber placed and fixed at right angles to the stock, the two are kept square by a timber brace. A batten fixed to the wall below the ceiling acts as a guide for the running mould.
A muffle is a piece of timber or steel placed over the profile of the stock. An approximate shape of the cornice is formed using the muffle leaving enough clearance for a top coat of gauge setting stuff.
A run cornice was well suited to lath and plaster ceilings which are seldom used nowadays. Running a cornice in-situ is a messy untidy way of working. A run cornice usually relies on one form of fixing which is the adhesive bond between the cornice and the wall/ceiling, if this bond fails a cornice will fall down. Where short lengths of wall will not accommodate the length of the slipper on the running mould other methods are required to form the cornice which is then difficult to fix as it is a solid lump.
In today’s working environment there is not enough of this type of work available for sizeable numbers of plasterers to familiarise themselves with, or become proficient in this type of work.
Many fibrous plaster bench made profiles are available, but more often than not these are a poor imitation of the real thing as they will usually avoid undercut sections which make the cornice difficult to remove from the reverse moulds used in fibrous plaster production. Companies also tend to avoid weak or thin sections which make the cornice difficult to cut successfully and transport without breakage.
Restoration of Victorian Plaster Cornice
It is not practical to refurbish and restore every period house using materials that were used originally. Time, expense, craftsmanship and availability have to be considered.
Substitute materials can be used to achieve the same effect and keep costs in check to produce a finished article which, when decoration has been applied, cannot be told apart from the original.
At White Chapel Art Plaster we have developed our own methods of replicating and/or matching traditional run in-situ Victorian cornice. Fibrous plaster casts are made in our shop, brought to site and expertly fixed in place by our fibrous fixers. These products are much more cost-effective and longer lasting, fireproof and have two forms of fixing to ensure the cornice stays in place and doesn’t move or crack.