238 PAINTING MATERIALS
Mrs Merrifield points out (I, cxi) that Eraclius described painting on plain leather
stretched on a panel and that Marco Rizzi is said to have painted on kid skins.
As late as the XVII century there is written evidence for a survival of this
practice. The MSS of De Mayerne (Berger, IV, 211) refer to a color for painting
leather and also to a ground for painting on leather (Berger, IV, 277).
Lime Plaster (see also Plaster). Since prehistoric times lime has been one of
the most important plastering materials. Pure lime itself or e neat * lime is not
used except for very thin surface washes and then it is usually mixed with salt,
glue, or casein. In thick layers, neat lime shrinks and cracks. To avoid this, it is
mixed with sand, finely crushed stone (frequently marble), or fibrous, organic
matter. Magnesia (MgO) is a frequent impurity in limes, and the so-called
* magnesia limes ' are derived from dolomitic limestones. Cowper (p. 16) says that
Italian limes contain up to 40 per cent magnesia. Magnesia limes slake more
slowly than high calcium limes. They expand less on slaking and shrink less on
drying and they set more slowly. A high magnesia lime is not considered a good
lime.
Lime is calcium oxide (CaO). It is prepared by heating one of the various forms
of calcium carbonate (CaCO3), limestone, marble, or chalk until nearly all of the
carbon dioxide is driven off, CaCO3 ^ CaO + CO2 T • When freshly prepared,
the product is called * quicklime * or * caustic lime,' for it readily combines with
water and decomposes organic matter. When quicklime is exposed to the air for
some time, it becomes air-slaked for the above reaction is reversible and the lime
combines with atmospheric carbon dioxide and returns to calcium carbonate.
When water is added to quicklime, the two combine vigorously with the liberation
of much heat. The product formed, calcium hydrate, is the slaked lime so im-
portant in the preparation of mortar, CaO + E^O —> Ca(OH)2. In fine mortars
and plasters it is essential that lime in this hydrated condition should be thor-
oughly seasoned, even over a period of months, before it is used. Calcium hydrate
combines with atmospheric carbon dioxide to form calcium carbonate, Ca(OH)a
+ COa —» CaCOa + H2O. This last reaction is essential to the setting of mortar.
There are two stages in the setting of lime plaster. The first is the drying out
of the water and the second is the subsequent hardening by chemical action be-
tween the calcium hydrate and the carbon dioxide of the air, resulting in the for-
mation of calcium carbonate and the liberation of a further quantity of water
which evaporates. To the formation of a coherent mass of crystals of calcium
carbonate—practically limestone—is, then, attributed generally the strength of
old plaster. This process of carbonation is excessively slow and generally in-
complete. Minute traces of quicklime were found in the pure lime plaster on the
walls of the palace at Knossos, in Crete, which had been exposed freely to the
atmosphere for about 3500 years; and free lime is found in the plaster used in the
Pyramids. In 1911 it was reported that free lime was present in most of the samples
of mortar taken from St Paul's Cathedral, London, and as much as 20 per cent