This damage has been caused by water pouring in through the roof over a number of years and dry rot taking hold in the right hand corner
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I first had to remove all the loose plaster and rotten laths
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Then the cornice was cut square through a cross section and a profile scribed onto sheet metal. This is then cut out and filed down to the required shape. A running mould can now be constructed by mounting the profile using screws onto a wooden stock cut to the same shape but a few mm smaller so as the metal profile overhangs the stock. This in turn is fixed perpendicular onto a horse, a length of wood long enough to be stable but short enough to get close into the internal angle to keep the mitre join to a minimum. The stock and horse are braced together to keep them at right angles, the brace also acts as a handle when running the cornice. A rebated strip of wood is fixed to the under side of the horse, the possition of which is determind by the thickness of the running rule which is a wooden baton fixed to the wall and used as a guide on which the running mould is run. See top right of tools pic for running mould
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The wall has been floated plumb with lime plaster and for practical reasons expanded metal has been used to replace the rotten lath and a light weight haired lime based plaster used to float the ceiling square with the wall. Both wall and ceiling have been keyed with a devil float but where the cornice is to run they have been keyed with a scratcher to give a deeper key for the heavier cornice
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The running rule is now fitted to the wall so as when the mould is run its bottom edge meets the bottom edge of the existing cornice.
Before the first rough coat of the cornice is run the profile has a muffle fitted. This can be a sheet of 2-3mm ply cut to the same shape as the profile only slightly larger so as the muffle overhangs the profile leaving room for the finish coat after the muffle has been removed. Depending on the depth of the cornice to be run a series of muffles may be needed so as to build up the cornice in a number of coats, each coat being keyed and set before the next is applied. A muffle can also be made using neat casting plaster moulded over the profile and when set cut back to within a few mm of the profile.
Gauged course stuff is applied to where the cornice is to be run using a laying on trowel.
The muffled running mould is now run the length of the wall as far into the internal corners as can be easonably achieved, ruling off any excess plaster. Any hollows are filled and the mould run again until all the hollows are filled. When the plaster has stiffened up i is keyed with a comb scratcher and allowed to set. after which the mitres (internal corners) are rulled in. This is done in the same plaster and using the set plaster of the run cornice as a screed and with the aid of a joint rule (see tools pic. They look like giant Stanley blades in centre of pic.) the excess is ruled off by drawing the joint rule up the profile of the cornice keeping it level as it is drawn up and coming out from the internal angle so as to form a true mitre. When using the existing cornice as a screed to form the opposite angle the rough coat is cut back a few mm so as to leave room for the finish coat to finish flush with the existing. These are also keyed with a comb scratcher and left to set.
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The muffle is removed leaving the true profile and a finish coat of gauged lime putty is applied in the same way.
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When the mitres are ruled in the ceiling and walls are finished with fine settng stuff
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I first had to remove all the loose plaster and rotten laths
Then the cornice was cut square through a cross section and a profile scribed onto sheet metal. This is then cut out and filed down to the required shape. A running mould can now be constructed by mounting the profile using screws onto a wooden stock cut to the same shape but a few mm smaller so as the metal profile overhangs the stock. This in turn is fixed perpendicular onto a horse, a length of wood long enough to be stable but short enough to get close into the internal angle to keep the mitre join to a minimum. The stock and horse are braced together to keep them at right angles, the brace also acts as a handle when running the cornice. A rebated strip of wood is fixed to the under side of the horse, the possition of which is determind by the thickness of the running rule which is a wooden baton fixed to the wall and used as a guide on which the running mould is run. See top right of tools pic for running mould
The wall has been floated plumb with lime plaster and for practical reasons expanded metal has been used to replace the rotten lath and a light weight haired lime based plaster used to float the ceiling square with the wall. Both wall and ceiling have been keyed with a devil float but where the cornice is to run they have been keyed with a scratcher to give a deeper key for the heavier cornice
The running rule is now fitted to the wall so as when the mould is run its bottom edge meets the bottom edge of the existing cornice.
Before the first rough coat of the cornice is run the profile has a muffle fitted. This can be a sheet of 2-3mm ply cut to the same shape as the profile only slightly larger so as the muffle overhangs the profile leaving room for the finish coat after the muffle has been removed. Depending on the depth of the cornice to be run a series of muffles may be needed so as to build up the cornice in a number of coats, each coat being keyed and set before the next is applied. A muffle can also be made using neat casting plaster moulded over the profile and when set cut back to within a few mm of the profile.
Gauged course stuff is applied to where the cornice is to be run using a laying on trowel.
The muffled running mould is now run the length of the wall as far into the internal corners as can be easonably achieved, ruling off any excess plaster. Any hollows are filled and the mould run again until all the hollows are filled. When the plaster has stiffened up i is keyed with a comb scratcher and allowed to set. after which the mitres (internal corners) are rulled in. This is done in the same plaster and using the set plaster of the run cornice as a screed and with the aid of a joint rule (see tools pic. They look like giant Stanley blades in centre of pic.) the excess is ruled off by drawing the joint rule up the profile of the cornice keeping it level as it is drawn up and coming out from the internal angle so as to form a true mitre. When using the existing cornice as a screed to form the opposite angle the rough coat is cut back a few mm so as to leave room for the finish coat to finish flush with the existing. These are also keyed with a comb scratcher and left to set.
The muffle is removed leaving the true profile and a finish coat of gauged lime putty is applied in the same way.
When the mitres are ruled in the ceiling and walls are finished with fine settng stuff