Making a test panel
You will need:
wooden tray, 40 x 40 cm, 2 cm deep (the base is Promatex, a moisture absorbing sheet material
measuring jug, 1 liter
plasterer’s trowel, 15 cm approx.
work panel, 80 x 80 cm approx.
water and detergent for cleaning equipment and brushes
Plaster base layer:
crushed stone fines, 800 cc
sharp sand, 800 cc
limepit slaked lime, 300 cc (X)
water, 75 cc (depending on sand moisture content)
Second plaster layer:
Crushed marble coarse, 800 cc
crushed marble fine, 800 cc
limepit slaked lime, 400 cc (X)
water 100 cc
Top plaster layer:
crushed marble fine 700 g
limepit slaked lime, 230 cc (X)
water, 75 cc
The limepit slaked lime has been stored under water in a pit for at least 7 years to give a jelly-like consistency.
Laying down the plaster base layer
Have all ingredients, tools and work panel (e.g. water resistant plywood) ready to hand. Moisten the tray with 350 cc water, or until the base is saturated. Mix the limepit slaked lime to an homogeneous mass. Use the trowel to mix all ingredients on the work panel into a smooth mass. Fill the tray to 1 cm under the rim and smooth the surface with the trowel. Leave to stand for approx. 12 hours. Smooth on the 2nd layer. Do not leave for longer than 12 hours or the next layer will not bind.
Laying on the 2nd plaster layer
Mix all the ingredients into a smooth mass. You might not need to use all the water. fill the tray to 3 mm under the rim. Allow to stand for 60 minutes.
Laying on the top plaster layer
Mix all ingredients into a smooth mass and fill the last 3 mm of the tray. Make as smooth as you can with the trowel. You will need some patience. When the layer is smooth, leave to stand 1 hour before painting.
The pigment is made up with water or lime water (a little slaked lime is added). Not all pigments resist alkalis. Slaked lime is an aggressive agent. It rapidly bleaches some pigments, and causes others to disappear entirely. Preferred pigments are: earth colours, cadmium (X), manganese pigments, cobalt pigments, ultramarine pigments and emerald green.
Safety goggles and gloves
Use goggles or a facemask to protect against splashes. Avoid contact with skin and eyes. Should splashing occur, rinse affected area copiously with water.