The manufacture of printing-ink or color is very difficult and dangerous on a large scale. I counsel all to take lessons from a book printer when he makes it.
The varnish must be prepared in the open, far from buildings, because of its combustibility. The best utensils and skilled workmen are required, because otherwise terrible accidents may occur, and even life be lost through explosion of the copper receptacle. Whoever does not require as much as one or more hundredweight of varnish in a year, would better buy it from printers or make only a small quantity, one or two pounds, and in an open vessel. For this purpose I will describe the process.
One, or at most two pounds of good old but not rancid linseed oil are poured into a clean iron pan which has a long, strong handle and is so large that the oil takes up only one half or, better, one third of the space. This is heated over a good fire till it burns, which is facilitated by applying flame to it. Oil that is too new has much water and other impurities that make it froth and run over. In that case the oil must be poured into the pan only in small quantities, when one must take great care to avoid spattering. As soon as the oil burns, the pan is removed from the fire and placed in a safe spot. If it is hot enough, it will continue to burn. It must now be stirred from time to time with an iron rod. Usually the flame increases under this stirring, but sinks again immediately at its cessation. So long as it does this, there is no danger that the flame cannot be easily extinguished if need be. But when it begins to continue burning with a great flame after the stirring stops, and at the same time to bubble and froth, it is high time to cover the pan with a close lid and leave it covered till the oil no longer takes fire when exposed again to the air. Then a dry knife is introduced and as much oil removed as will adhere to its point. If it does not permit itself to be pulled into long threads when cool, but is too thin, it must be heated again until it gets the required consistency.
A good varnish dries very readily of itself, and it is not only unnecessary but inadvisable to mix a drier with it, as varnish so treated is too likely to off-set on the stone.