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Forum URL: http://www.truefresco.com/cgidir/dcforum/dcboard.cgi
Forum Name: Fresco Workshop Program
Topic ID: 19
Message ID: 10
#10, RE: Fresco painting PROBLEMS explored!!!
Posted by Gary sculptari on 31-May-04 at 07:35 PM
In response to message #9
I'm coming in late here - but I can help you.

You have to be aware of different sizes of sand. Silica makes the strongest mix, but a calcium carbonate sand will be weaker, fluffier, and therefore more absorbent. Calcium carbonate is marble or limestone. I checked on the net - Moncton has a huge limestone crushing plant, one of the tenth largest in Canada, and also silica. You should be able to get everything you need from Lafarge - including lime - just make sure its high calcite and air classified. Just like in sculpture, where the armature is the key, sand is as important as the putty. All lime has a chemical analysis - you want highest calcium as possible - any building lime will be close, sometimes though real cheap agricultural grades are out there - but they are never sold for building trades.

The final layer should be almost fifty/fifty putty and fine white sand - I use 'o/o' size in dolomite limestone - a softer sand than silica - this size is like table salt. Variances either way will create a sandier, textured finish, or a gummier finish which will crack slightly. Experience is the teacher. When you call Lafarge, get a gallon of concrete bonder too, to use between layers - it is a modern variation which works extremely well - but don't let it contaminate your surface - it makes a gloss.

SO like painting light to dark, warm to cool, in plaster you work lean (sandy/coarse/rough/thick/strong) to fat (lime rich/smooth/ thin coat). The first coat I use silica sand, the middle marble sand, the final dolomite/limestone sand - and three coats are more than adequate - expecially if the first is reinforced with galvanised steel mesh 'lath' (also from or directed to by lafarge).

I have never had a problem with drying time - but I have also found an acceptable 'secco' technique using potassium silicate (the ingredient of 'Keim' paints) - and also made in Canada, and fresco pigments. True fresco on fresh lime and potsil over 'green' lime are a marriage made in heaven.