Davidstacey42
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I've followed this post with great interest because up until now, I thought I was the only person to consider using Chalk in place of sand. The (largely intact and presumed original) plaster in the bedroom of my timber-framed cottage (dendro-dated to 1621) appears to have been applied in a single coat, about a quarter of an inch thick, straight onto an earthy background which I believe is the top surface of the daub on the wattle panels. The plaster and a thin layer of daub has been hacked away in places to reveal the timber studs but these were obviously originally plastered over. When I moved in, the walls looked very lumpy but once I removed the woodchip (which took with it umpteen coats of vinyl silk emulsion, limewash etc) and the odd skimmed patch of pink gypsum, I discovered fine flat plaster with the texture and colour of polished marble. The plaster is hard as nails on the surface but powdery/crumbly underneath - there's no evidence of sand in it but plenty of short dark brown hair. It is quite yellow in colour and the other day, I found a perfect imprint of a tiny leaf (not sure if by accident or design).
I've tried using sand/lime mixes and pure lime putty to do repairs but the comparatively hard set just didn't blend in with the old stuff to my eye. It then occurred to me that by 'cutting' pure lime putty with chalk, I might get the weaker set I was after. I experimented with different proportions and got good workability and adhesion but was still plagued with shrinkage. Trawling the net, I stumbled across Venetian plastering where the plaster seems to consist of nothing more than lime putty, marble dust and linseed oil. Why not, I reasoned, try adding some linseed oil to the mix and see what happens. The result was a complete revelation. The resulting plaster is very sticky but an absolute joy to work with. The initial set is slightly slower than with lime/sand mixes because the linseed oil causes a skin to form (like glazier's putty) which prevents rapid drying-out. The following day, I polish up with a clean trowel, lubricating with a water mister (you have to be a bit careful as the surface can tear if the trowel picks up). The finish is super-fine and texture-wise, a very good match to my old plaster. I'm still very much at the experimental stage as I want to see if my mix stands the test of time but if it does, there will be no going back to lime/sand for me. The other great advantage is that this type of plaster weighs far less than those containing sand.
I've tried using sand/lime mixes and pure lime putty to do repairs but the comparatively hard set just didn't blend in with the old stuff to my eye. It then occurred to me that by 'cutting' pure lime putty with chalk, I might get the weaker set I was after. I experimented with different proportions and got good workability and adhesion but was still plagued with shrinkage. Trawling the net, I stumbled across Venetian plastering where the plaster seems to consist of nothing more than lime putty, marble dust and linseed oil. Why not, I reasoned, try adding some linseed oil to the mix and see what happens. The result was a complete revelation. The resulting plaster is very sticky but an absolute joy to work with. The initial set is slightly slower than with lime/sand mixes because the linseed oil causes a skin to form (like glazier's putty) which prevents rapid drying-out. The following day, I polish up with a clean trowel, lubricating with a water mister (you have to be a bit careful as the surface can tear if the trowel picks up). The finish is super-fine and texture-wise, a very good match to my old plaster. I'm still very much at the experimental stage as I want to see if my mix stands the test of time but if it does, there will be no going back to lime/sand for me. The other great advantage is that this type of plaster weighs far less than those containing sand.