Making Oil Varnish
Information and guide to making oil varnish for painting.
The varnish-maker puts a hundred pounds of resin in a kettle over a hot fire, and when it is melted it is at about the temperature of melted lead; then he puts in hot linseed oil. If he puts in oil equal in weight to the melted resin (which has lost between a fifth and a fourth of its weight in melting), he will make a hard, lustrous varnish, suitable for tables and other furniture; if he puts in twice this amount, he will have a varnish suitable for general interior house-varnishing, elastic and durable, but not quite hard enough for furniture; if he puts in three times as much oil as resin, he will make a very elastic, durable varnish which will get hard out of doors, but indoors will harden too slowly to be agreeable to use. After the oil is put with the resin the compound is cooked several hours until it has thoroughly united; then turpentine is added in sufficient amount to make it thin enough to use, for it is evident that the resin will have thickened the oil so that it would not brush out.
It will be seen that the first and most important cause of differences in varnishes is the relative proportion of resin and oil. It is also true that if the varnish is not cooked enough to thoroughly combine the ingredients, it will lack durability; and also that it may be partly decomposed by overheating. If it is just right, it will repel water, and not be affected by it; but if not well made, or if made of poor materials, it may absorb water and be injured if subjected to its action, which is certain to take place. One of the most important and easily made tests for varnish is to varnish a board well with it and let it dry well; then put a wet sponge on it over-night. If it is bright and clear next morning, it is a good varnish of its kind, so far as durability is concerned;, but if it turns white, it shows that it has absorbed water; and if it remains white after it has dried out, it shows that the water has dissolved out part of it. Such a varnish should not be used on wood-work about a house.
It is not a good sign to have a varnish dry very quickly, as it may indicate that it is overloaded with drier, and that will injure its durability. A good house-varnish should be dry to the touch over-night, at summer temperature in dry weather, but it should stand a week before receiving another coat.
Next Page: How to Apply Varnish.
This is Making Oil Varnish.
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