MANGANESE.
377
refraction, they anneal the glass which has stiffened in thefurnace, then break it, and form the pieces to proper size orshape, by grinding, &c. For common green, or blackish-green glass, every kind of siliceous earth is proper; but fora beautiful white transparent and perfectly clear glass, thepurest sand or quartz, as well as the most pure of alkalinesalts, are requisite. Even then the glass will be far frompurity, unless a proper portion of manganese be added tothe frit. 1 *
However, to proceed with our original intention of col-lecting such relevant, and what we believe to be pertinenttestimonies of ancient practice, and by the way, to gatheruseful and ingenious opinions we find variously diffused withregard to its manufacture.
\Ve proceed to state, that although the art of manufac-turing glass may have arisen from accident, like that men-tioned by Pliny , lib. xxxvi. 26. § 25, our present citedauthor 3 conceives glass might have originally been producedby accident; but his chief object is to inquire by what meansthe ancient artists w'ere first induced to apply manganese inits composition, as it is very evident they did: this assuredlydeserves inquiry, considering that it appears to be a mine-ral, whose outward appearance does not indicate any proxi-mity or relationship with so transparent a substance. Thatthe ancients discovered that iron stone was capable of com-municating a great variety of colours and rich tints to glass,with that it is conjectured they produced their valuableblues, reds, purples, &c. Now', as manganese has been,and might very naturally be taken for a species of this stone,and from using it sparingly, they discovered its valuableproperty in giving a perfect transparency to the article;w’hen it is used in larger proportion, it Will give to glass aviolet colour, something similar to the amethyst.
From what has been shown, it has been thought that theapplication of manganese, in the manufacture of glass, wasthe produce of accident; because it is justly noticed, that
1 This term is perfectly understood by glass-makers and writers on the article :it denominates a certain mixture of sand, or siliceous earths, and alkaline salt. Itappears to have been an Italian term, and to have come from the kitchen, fritta ,derived fronijntto, signifying something broiled or roasted. Thus it is used inThomas Norton 's poem, Crede mihi sive Ordinate, written in 1 477, and oftenprinted.
Durior species (vitri) vocatur/reton,
Ex vitrorum Iracturis id evenit,
TinctuTa smaltorum vitriariorumNon penetrabit iJJud, ut referunt.
3 Beckmann.