327. 5. Rezin, or Chio Turpentine may be added, till the Varnish has obtained the desired Thickness.
327. 6. It must likewise be observed, that Litharge rots the Silk: therefore Trials must be made without the Use of Litharge.
327. 7. The longer the raw Linseed-Oil remains on the unslacked Lime, or Umber, the sooner will the Oil dry, after it is used.
If some Months; so much the better. Such Varnish will set, i. e. will not run, but keep its Place on the Silk, in four Hours.
The Silk may then be turned, and varnished on the other Side.
328. on gum mastic, sandarac, seed-lac, shell-lac, and copal.
328. 1. Gum Mastic dissolves, without pounding, by adding a few Drops of Oil of Vitriol: so do Gum Sandarac, and Gum Copal, when finely pounded and sifted.
328. 2. Gum Sandarac, and Gum Mastic are great Driers of themselves: and may be substituted for Litharge.
328. 3. The Mastic dissolved in the Oil of Vitriol, gives a sweet Smell to the Varnish.
328. 4. Sandarac will soon grow dusk in the Fire: it melts into a transparent Liquor.