"Eight pounds of Copal takes in general from sixteen to twenty minutes in fusing, from the beginning till it gets clear like oil; but the time depends very much on the heat of the fire and the attention of the operator. During the first twelve minutes while the gum is fusing the assistant must look to the oil, which is to be heated at a separate fire in a copper pot, large enough to contain double the quantity required. The oil should be brought to a smart simmer, for it ought neither to be too hot nor too cold, but in appearance beginning to boil, which the assistant is strictly to observe, and when ready, call to the maker, then immediately each take hold of one handle of the boiling pot and carry it to the ash-bed, the maker instantly returning to the gum-pot, while the assistant ladles the hot oil into the copper pouring jack, bringing it and placing it at the back of the gum-pot until wanted.
A thick piece of old carpet, free from holes, should be kept at hand in case the gum-pot should take fire; should this happen, let the assistant throw the piece of carpet quickly over the blazing pot, holding it down all round the edges; and in a few minutes the fire will be smothered.
After the oil has been mixed with the gum, a brisk, strong fire should be kept up, until a scum or froth rises and covers all the surface of the contents, when it will begin to rise rapidly. Observe when it rises about two-thirds the height of the pot, carry it from the fire, and set it on the ash-bed, or brick-stand, stir it down again; and if driers are to be added, scatter in a few by a little at a time; keep stirring, and if the frothy head goes down, put the pot on the fire, and introduce gradually the remainder of the driers, always carrying the pot to the ash-bed when the froth rises about two-thirds the height of the pot. In general, if the fire be good, all the time a pot requires to boil from the time of the oil being poured in, is about three-and-a-half or four hours; but time is no criterion for a beginner to judge by, as it may vary according to the weather, the quality of the ingredients, or the heat of the fire; therefore, about the third hour of boiling, try it on a bit of glass, and keep boiling it until it feels strong and stringy between the fingers, as before mentioned.
The foregoing directions are, with very little differences, to be observed in making all sorts of copal varnishes, excepting the quantities of oil, gum, etc, a few of which will be now added.
Copal varnish for fine paintings etc. Fuse 8 lbs. of the very cleanest pale African gum copal, and, when completely run fluid, pour in two gallons of hot oil; let it boil until it will string very strong; and in about fifteen minutes, or while it is yet very hot, pour in three gallons of turpentine, got from the top of a cistern. Perhaps during the mixing a considerable quantity of the turpentine will escape, but the varnish will be so much the brighter, transparent, and fluid; and will work freer, dry quickly, and be very solid and durable when dry. After the varnish has been strained, if it is found too thick, before it is quite cold, heat as much turpentine and mix with it as will bring it to a proper consistence.
Artist's virgin copal. From a select parcel of scraped African gum copal, before it is broke, pick out the very fine transparent pieces, which appear round and pale, like drops of crystal; break these very small; dry them in the sun, or by a very gentle fire. Afterwards, when cool, bruise or pound them into a coarse powder; then procure some broken bottles or flint-glass, and boil the same in soft water and soda, then bruise it into a coarse powder, like the gum; boil it a second time, and strain the water from it, washing it with three or four waters, that it may be perfectly clean and free from grease or any impurity, dry it before the fire, or upon a plate set in an oven. When thoroughly dry, mix 2 lbs. of the powdered glass with 3 lbs. of the powdered copal; after mixing them well, put them into the gum-pot, and fuse the gum; keep stirring all the time; the glass will prevent the gum from adhering together, so that a very moderate fire will cause the gum to fuse. When it appears sufficiently run, have ready three quarts of clarified oil, very hot, to pour in. Afterwards let it boil until it strings freely between the fingers. Begin and mix it rather hotter than if it were body varnish, for, as there is but a small quantity, it will be sooner cold; pour in five quarts of old turpentine, strain it immediately, and pour it an open jar, or large glass bottle; expose it to the air and light, but keep it both from the sun and moisture until it is of a sufficient age for use. This is the finest copal varnish for fine paintings.
Cabinet varnish. Fuse 7 lbs. of very fine African gum copal, when well dissolved pour in half a gallon of pale clarified oil; and when clear mix with it three gallons of turpentine; afterwards strain it, and put it aside for use. This, if properly boiled, will dry in ten minutes; but if too strongly boiled, will not mix at all with the turpentine; and sometimes, when boiled with the turpentine will mix, and yet refuse to amalgamate with any other varnish less boiled than itself; therefore it requires a nicety which is only to be learned from practice. This varnish is very apt to chill all other oil varnishes to which it may be added, and is principally employed as a quick drying varnish for the occasional use of japanners, cabinet and coach painters. Cabinet varnish is, however, more generally made with anime than copal.
Best body copal varnish, used for the body parts of coaches and other objects intended for polishing. Fuse 8 lbs. of fine African gum copal, add 2 gallons of clarified oil; boil it very slowly for four or five hours, until quite stringy, and mix it off with 3 1/2 gallons of turpentine.
The above varnishes being made of the finest copal without driers are the palest and best of the copal varnishes, possessing great fluidity and pliability, but they are rather slow in drying, and retain for months so much softness that they will not polish well, until they give out a moisture and become hard; after which they are very durable. When paleness is not of primary importance a second quality of gum is used, and when the varnish is required to dry quickly, sugar of lead or white copperas are introduced as driers, either singly or combined, in the proportion of from half a pound to one pound to each of the quantities above quoted, but driers are always injurious to the colour, brilliancy and durability of varnishes. When a varnish is required that will dry quick and hard without driers, gum anime is substituted for the copal, but it is less durable and becomes darker by age. Frequently anime varnish is mixed with copal varnish by the maker while both are hot, in different proportions according to the quality required; one pot of anime to two of copal being used for a moderately quick drying body varnish of good quality; and two pots of anime to one of copal for a quicker drying body varnish of common quality.