WHITE.
For this an article called China clay is used; also, for some purposes, the common pipe-clay.
GUM.
Of all the varieties of gum, there is but one that is of any use to the marbler, and that is called gum-tragacanth or gum-dragon. Too much care cannot be exercised in the choice of this article, as much of the excellence of the work depends upon it. It should be large, white, and flaky. Occasionally there will be found some very good in small white flakes; but let that in dark brown lumps be rejected at once, no matter at what price it may be offered. If used at all, it would only do for the most common kind of work; but there is, in reality, no saving in an inferior article, as one pound of a really good sort will go as far as two of a bad, and produce a far more satisfactory result. Good gum ought to dissolve in cold water; it requires at least forty-eight hours soaking, being well stirred about at intervals; but some gums take longer to dissolve. Good gum will produce a smooth surface, but bad gum will often yield a rough one, which is inimical to the purpose. Again, some will give a smooth surface, and yet possess no strength; the colours will flow well upon it and form properly, and, when the paper is taken off, will look, at first, very beautiful, but upon looking at it five or ten minutes after it has been hung up, the colours will be found running off, causing indescribable annoyance and mortification.
DIRECTIONS FOR PREPARING THE GUM.
Procure a large earthen pan, glazed on the inside, capable of containing from eight to twelve gallons of water. Put therein one pound of gum-tragacanth, and on it pour about two gallons of soft water. Stir it every few hours with a clean birch broom kept expressly for the purpose, breaking the lumps and adding more water as it thickens or absorbs that previously put in. In about forty-eight hours you may venture to use it; but seventy-two hours would be better. Some gum is all the better for a longer period, as, although a considerable portion of the gum may be dissolved, yet the best properties of it are not extracted till the whole is dissolved. It must be strained through a fine hair sieve before using, and if any lumps remain, put them back into the pan until they are all dissolved.
OF LINSEED.
It is possible to marble some patterns on mucilage of linseed, but it is a very objectionable vehicle to work upon, and can never be made to produce a satisfactory result. It is made either by boiling one quart of linseed in six or eight gallons of water, or by pouring the boiling water upon the linseed and stirring it until it extracts the mucilaginous properties of the seed; but it very soon decomposes or turns to water.