The lime should be thoroughly slaked, so as to deprive it of its caustic properties, but it does not follow that it should be twenty or thirty years old. Lime can be kept in a slaked state and skimmed until it almost ceases to be lime at all, and this worn-out material is unfit for fresco. Then the sand should be gritty and hard to the touch. Clean river-sand collected in a granite country is very good; ground lava is used by modern Italian fresco-painters.
I do not know where the sand supplied to the fresco-painters of Westminster Palace came from, but it was a great deal too fine and soft to the touch.
The older and more worn-out the lime is, the sharper and more tenacious ought to be the sand.
Having got some well-slaked but not worn-out lime and some good hard sand, the mortar that is required for the day’s use should be made fresh every day, or at least as often as twice a week.
When I was painting some frescoes at Islington, I got my intonaco from a man who had had great experience. Instead, however, of sending me the lime and sand separate, he sent me about twenty small barrels of ready-made mortar. My work took me nearly two years, and every morning my plasterer had to go with a pick-axe and hack a piece of dry mortar out of the barrels.
This he beat up with water and spread it for my day’s work, smacking his lips as if he had got a most delicious compound on his trowel. I knew no better then, but now I am surprised, not that the frescoes should be decaying, but that the decay should not be more rapid. Improper colors and the omnipresent gas may have had something to do with the decay of all frescoes painted in London, but from experience I can assert with confidence that the main cause has been the weakness of the lime and sand.
We will suppose in our imaginary decoration that we don’t fall into this mistake, that we get lime of the proper strength and clean granite sand. We will also suppose that we don’t get a dozen barrels of mortar made up, but have our intonaco mixed fresh every other day.
The first thing to be painted is the sky or background, whatever it may be. We mark out on the wall with charcoal the extreme extent of this background. We don’t trace the outline of the heads, but make our black mark well beyond where this outline should be.
The plasterer ought to be an early riser, so that by nine or ten o’clock when we arrive we may find the mortar all ready for us, even in surface, and tolerably firm or “set” as it is called.
I never could get an English plasterer to throw the mortar against the wall, as is done by Italian and French workmen. When spoken to about it he always seemed to think he ought to know his own trade best, or perhaps the Union forbids him to make the mortar stick too close.