When making spirit varnishes for polishing furniture etc., the gums and resins (colophony, sandarac and shellac etc.) are dissolved in ethyl alcohol (ethanol) – or more acceptably, for safety reasons these days – Industrial Methylated Spirit (IMS or ‘meths’). Meths is also known in some parts as denatured alcohol (DNA).
Aside from the added denaturants that make methylated spirit unpalatable, meths purchased from hardware shops or supermarkets may legally contain up to 43% water as well. Water is miscible with alcohol and is a profitable means of cutting meths without greatly affecting its usefulness for most domestic purposes.
Any spirit that won’t, or is slow to dissolve shellac, almost certainly contains a detrimental percentage of water. It is prudent to read the labels on meths containers to ascertain if the contents have been adulterated with water and, if so, to what degree. Newly bought spirit labelled as being 95% or greater purity will dissolve good quality, fresh shellac quite adequately.
The age-old method of testing spirit’s fitness for varnish-making is quite straightforward:
Rectified Spirits are made from Wine, or Sugar, or Mault Liquors; or from Cyder, Perry or Moloſſos; or from the common Spirits made of thoſe things by Rectification. They are for the Diſſolution of Roſins or Gum Roſins and therefore if not highly rectified, are unprofitable for theſe Uſes.
Now, to know whether your Spirit is good or no, you muſt put ſome of it into a Spoon, and put a little Gun-pouder to it; then ſet the Spirit on Fire, if it burns all away and fires the Gun-pouder after it, it is good, and will diſſolve your Gums; otherwiſe, not.[i]
(Hence the term gunpowder-proof or ‘proof’ used to describe alcohol content.)
Not nearly as good fun as proving meths with gunpowder, but equally efficacious, is this simple method: Pour a little meths into a container, add a few drops of mineral turpentine and then slosh it around. If the contents remain clear, the meths is perfectly all right, but if it turns cloudy, the meths contains an undesirable quantity of water and is only good for cleaning one’s spectacles.
[i] William Salmon, Polygraphice, or, The Arts of Drawing, Engraving, Etching, Limning, Painting, Vernishing, Japaning, Gilding, &c., eighth edition, A. and J. Churchill, and John Nicholson, London, 1701, p. 857.