This section is from the book "Amateur Work Magazine Vol6". Also available from Amazon: Amateur Work.
The word "varnish" is understood to be derived from the Latin vitrinere, meaning to glaze or produce a glass-like surface. Of the greater variety of gums used in the making of varnish, shellac is the most useful for spirit varnishes. Although the annual consumption of gum shellac in this country is now about 5,000 tons, there are points about it not generally known or understood. It is not a resin in the strict sense of the word; i. e., it is not the simple juice of a tree, but results from the action of certain insects on the juice and contains several very peculiar resins.
Next to shellac sandarac ranks as the most valuable gum for spirit varnishes. As for the regular gums, although the list in books is a long one, practically all that varnish makers are interested in may be counted on the fingers of the hands, and this can be further reduced to four, viz., Zanzibar, Kauri, Manila and Da-mar. The impression prevails that great quantities of these gums are shipped to the American market, but such is not the case. The imports for the year 1905 were not much in excess of 13,000 tons, and of this about 50 per cent, was Kauri.
Zanzibar stands at the head, being the hardest of all gums, except amber, which need not be considered. It derives its name, as may be inferred, from the port of shipment, as indeed most other gums do, excepting, perhaps, Manila. We speak of all these hard gums as "fossils," because they are found in a fossilized condition in the ground, sometimes hundreds of feet below the surface. Zanzibar is dug out of the sands of the African desert, and the curious indentations which give this gum the appearance of goose skin are simply sand impressions. The Zanzibar gum is scarce and very expensive.
Next in point of costliness, but far in advance in point of usefulness to the varnish maker, are the New Zealand copals, commonly called "Kauri" gums. They range in color from a creamy white to a dark brown, and are so graded. Much of this gum is not available for use, and the assorting requires skill and care. The lower grades contain pitch and swamp gum, the former being taken from the forks of trees 100 feet or more above the ground. It is a soft, spongy mass, and it is extremely difficult to incorporate with the oils. Ninety per cent, of what is imported, however, may be classed as good hard gum, differing only in size, color and clearness.
Manila gum is a soft copal exported from the Dutch East Indies. It is in more or less demand, but varnish makers have no great use for it. However, a small quantity sometimes helps to give elasticity to harder gums, and occasionally it is used in spirit goods. For general use its greatest drawback is the difficulty of eliminating the pyroligneous acid, of which it carries quite a large per cent. There are many other varieties of gums in the same class as Manila, but they are not used to any considerable extent, perhaps 1,000 tons would cover the annual importation.
The blacks are a small line mostly used in baking or air drying japans and varnishes. Originally our supplies of asphaltum, which is supposed to be the product of decomposed animal and vegetable matter, came from the shores of the Dead Sea, and "Egyptian" continues to be one of the best grades. We now get a considerable quantity from both Trinidad and Barbadoes, and Cuba sends us an asphaltum that is densely black. In this country Colorado and Utah mine very heavily. There are other blacks besides asphaltum, but as a rule they do not interest the varnish maker-such as coal tar pitch, resin-pitch, candle-pitch, etc. There is more or less interest attached to all these crude materials which enter into the composition of varnish, but space will not permit of my dwelling on them at length. One other, however, I will mention-China wood oil. This is a much more expensive oil than linseed, and very hard to manipulate to get proper results, but is none the less a most valuable article for those varnish makers who have mastered its secrets. When worked in the same way as linseed oil it makes a harder, more elastic and more durable varnish.
As for the different grades of varnishes, the numerous catalogues which are issued show what a large variety of varnishes there are for sales purposes; but the classification may be considerably modified. Originally there were but two classes on the market-carriage varnishes and furniture varnishes. The introduction later of so many beautiful woods in building operations made an architectural line imperatively necessary. Outside of the above the manufacturers' lines and specialties make an almost endless list; yet they are all modifications of a general line to suit certain conditions and for the most part are obtained by blending.
All the better grades of varnish, no matter what the line, are made of selected gums and are specially prepared oils, with pure turpentine as a thinner. The cheaper grades will naturally carry poorer gum, and be thinned with either naphtha or part naphtha and part turpentine.
Speaking of naphtha and turpentine thinners, it may be interesting to know what the essential difference is. The turpentine varnish undoubtedly works easily, and it dries from the bottom up. With naphtha these features are reversed. Sometimes it forms a skin over the top, keeping out the oxygen, and so retarding the drying. Another serious fault with naphtha goods is that they do not flow as turps does. Both, however, are used merely as distributers, for neither stays where it is placed, but evaporates in due course. Most of the difference between the two liquids lies in the fact that turpentine carries a percentage of oils and naphtha does not. It is easier and better to thin with turps because it can be added at a higher temperature-350° to 360° Farenheit-and at this temperature the combination of gum and oil is more perfect.
There are many terms in the technology of varnish which convey no definite meaning to the outsider; yet they are full of suggestions as to the possibility for use of said varnish. For instance, the varnish maker speaks of "slack melt." By this it meant that the gum is melted in a covered kettle to a semi-liquid condition before the oil and thinners are added. The result is a large yield, good color, hard working and a false body. The batch takes a large quantity of thinners, which in itself causes a loss of gloss. The object is simply good color, or large yield, or both. There is more or les moisture left in this varnish, and if mixed with pigment it would be apt to liver. If it did not do that it would not mix well. So for this purpose the using of a "slack melt" is to be avoided.
An "open melt" is when the cover is left off the kettle, the object being to throw off as much moisture and copal oil (the oil that is in the gum) as possible. When this is not done the varnish is liable to "bloom," and the oil mentioned retards the drying.
For a "close melt" the cover is left on and a larger yield is produced, as most of the gum is retained in the kettle. This is used for the cheaper grades than the best. Good results are also obtained in a "close melt" by different manipulation. One way is to melt the gum to a liquid state before the oil is added. In doing this color and yield are sacrificed to a certain extent, but finer results are produced as to gloss, drying, freedom of working and wearing qualities. In all gum melting too strong a fire is to be avoided or the gum will be burned.
Zanzibar and other fossil gums are difficult to handle. Usually the heat varies from 550° to 640° for the harder gums. Japans (outside of the grinding varieties, which are made with shellac and gums) and liquid dryers are usually made by boiling lead and manganese with linseed oil, combining as much metal as possible with the oil, and driving off the oxygen with long sustained heat.
It is a truism that good varnish depends more on the makers than on the material. This has frequently been proven, since with the same material one man has made a good varnish and another a very poor one. Thoroughly competent men for this work are scarce. It requires brains, nerve, judgment and a perfect knowledge of materials-not merely the crude Material, but of the finished product, and what it has to accomplish. He must be weather wise also, for under certain circumstances the weather is a most important factor to consider. For instance, the atmosphere indicates posible mugginess, with little or no draught. He realizes at once that he must make up the strongest kind of a fire before he runs the kettle on, otherwise the contents will simmer, darken and spoil. Or again, he may wish to make a varnish of another class, which requires a bright, clear day, with not too much wind. The successful Varnish maker must be able to cope with all of these conditions. He must be a man of discernment, capable of perceiving possible danger or loss before the actual crisis arrives, and a man of resource, so that he can constantly meet, counteract and overcome any troubles which may arise to hinder the successful making of his varnish.-"American Exporter."