Kindle a fire in the furnace underneath, and manage the fire so that the oil shall gradually but slowly increase in heat for the first two hours; then increase the heat to a gentle simmer, and if there is any scum on the surface, skim it off with a copper ladle and put the skimmings away. Let the oil boil gently for three hours longer, then introduce, by a little at a time, a quarter of an ounce of the best calcined magnesia for every gallon of oil, occasionally stirring the oil from the bottom. When the magnesia is all in, let the oil boil rather smartly for one hour; it will then be sufficient. Lay a cover over the oil to keep out the dust while the fire is drawn and extinguished by water; then uncover the oil, and leave it till next morning; and then, while it is yet hot, ladle it into the carrying jack, or let it out through the pipe and cock; carry it away, and deposit it in either a tin or leaden cistern, for wood vessels will not hold it; let it remain to settle for at least three months. The magnesia will absorb all the acid and mucilage from the oil, and fall to the bottom of the cistern, leaving the oil clear, transparent, and fit for use.
First procure a gum pot, Fig. 3, or smaller, if required; then a three-footed iron trevet with a circular top, the feet 16 in. in length, and made to stand wider at the bottom than at the top, which is to be made so that the pot will fit easily into it. Place the trevet in a hollow in a yard, garden, or outhouse, where there can be no danger from fire; raise a temporary fire-place round the trevet with loose bricks, after the same manner that plumbers make their furnaces; then make up a good fire with either coke, coal, or wood-charcoal, which is far preferable; let the fire burn to a good strong heat, set on the gum pot with 3 lbs. gum copal; observe, that if the fire surround the gum pot any higher inside than the gum, it is in great danger of taking fire. As soon as the gum begins to fuse and steam, stir it with the copper stirrer, and keep cutting and stirring the gum to assist its fusion; if it feels lumpy and not fluid, and rises to the middle of the pot, lift it from the fire and set it on the ash-bed, and keep stirring until it goes down (meantime let the fire be kept briskly up); then set on the gum pot again, and keep stirring until the gum appears fluid like oil, which is to be known by lifting up the stirrer so far as to see the blade.
Observe, that if the gum does not appear quite fluid as oil, carry it out whenever it rises to the middle of the pot, and stir it down again, keeping up a brisk fire; put on the pot, and keep stirring until the gum rises.above the blade of the stirrer. Then the copper pouring jack is charged with boiled oil, and held over the edge of the gum pot; when the gum rises within 5 inches of the pot-mouth, the assistant is to pour in the oil very slowly until towards the last, the maker stirring during the pouring. If the fire at this time is strong and regular, in about eight or ten minutes the gum and oil will concentrate and become quite clear; this is to be tested by taking a piece of glass and dropping a portion of the varnish on it; if it appears clear and transparent, the oil and gum are become concentrated or joined together. It is now to be further boiled until it will string between the finger and thumb; this is known by once every minute dropping a portion on the glass, and taking a little between the forefinger and thumb; pinch it first, then extend wide the finger and thumb; if it is boiled enough, it will stick strong and string out into fine filaments, like birdlime; but when not boiled enough, it is soft, thick, and greasy, without being stringy.
It is a safe plan to have ready a thick piece of carpet large enough to cover the mouth of the boiling pot should it catch fire during the pouring. The moment it is boiled enough, carry it from the fire to the ash-bed, where let it remain from fifteen to twenty minutes, or until it is cold enough to be mixed; have at hand a sufficient quantity of oil of turpentine to fill the pouring pot, begin and pour out with a small stream, gradually increasing, and if the varnish rises rapidly in the pot, keep stirring it constantly at the surface with the stirrer to break the bubbles, taking care not to let the stirrer touch the bottom of the pot, for if it should, the oil of turpentine would be in part converted into vapour, and the varnish would run over the pot in a moment; therefore, during the mixing, keep constantly stirring as well as pouring in at the same time. Have also a copper ladle at hand, and if it should so far rise as to be unmanageable, let the assistant take the ladle and cool it down with it, lifting up one ladleful after another, and letting it fall into the pot.
As soon as the varnish is mixed, put the varnish sieve in the copper funnel placed in the carrying tin, and strain the varnish immediately; empty it into open-mouthed jars, tins, or cisterns; there let it remain to settle, and the longer it remains the better it will become. Recollect, when it is taken out, not to disturb or raise up the bottoms.
The choice of linseed oil is of peculiar consequence to the varnish maker, as upon its quality, to a great extent, depends the beauty and durability of the varnish. Oil expressed from green unripe seeds always abounds with watery, acidulous particles. The quality of oil may be determined in the following manner. Fill a phial with oil, and hold it up to the light; if bad, it will appear opaque, turbid, and thick; its taste is acid and bitter upon the tongue, and it smells rancid and strong: this ought to be rejected. Oil from fine full-grown ripe seed, when viewed in a phial, will appear limpid, pale, and brilliant; it is mellow and sweet to the taste, has very little smell, is specifically lighter than impure oil, and when boiled or clarified dries quickly and firmly, and does not materially change the colour of the varnish when made, but appears limpid and brilliant.