280 EARTHS. 



ty, and higher refractive power. It is the glass of our tables, of op- 

 tical instruments, and lustres. 



(b.) Crown Glass. 200 parts of good soda, (or pearl ashes,) 

 300 pure sand, 33 lime, 250 to 300 ground fragments of glass ; this 

 last addition is not essential ; or, by measure, fine sand purified 5, 

 best kelp, ground, 1 1 ; by weight, sand 200, kelp 330. Professor 

 Sweigger discovered that sulphate of soda might be used in the man- 

 ufacture of glass, and his proportions are, sand 100, dry sulphate of 

 soda 50, dry quick lime, in powder, 17 to 20, charcoal 4. There- 

 suit is a good glass ; the sulphate of soda, aided especially by the 

 charcoal, is decomposed, and its soda combines with the silica and 

 the lime aids in producing the vitrification. The materials of glass 

 are combined, in part, by a preliminary operation, called fritting, 

 performed in a furnace, by which sulphur and other volatile mat- 

 ters are expelled, previous to the full fusion, and the alkali is brought 

 into combination with the silica, so that it is not volatilized by a 

 higher heat. 



(c.) Broad glass. Soap maker's waste 2,* sand 1, kelp 1, mix- 

 ed, dried and fritted ; or, soap boiler's waste, 6 bushels, 3 of kelp, 

 and 4 of sand ; these form a pretty good broad glass. The materi- 

 als are calcined for 20 or 30 hours before fusion, and then it requires 

 12 or 15 hours to melt them into perfect glass. 



(d.) Plate glass 300 Ibs. sand, 200 soda, 30 lime, 32 oz. man- 

 ganese, 3 oz. azure, and 300 Ibs. fragments of glass ; or pure sand 

 43, dry soda 26.5, pure quick lime 4, nitre 1.5, broken plate glass 

 25 = 100, from which 90 parts of good plate glass may be obtained. 



(e.) Bottle glass. Common sand,f 100 parts, 30 of varec or 

 coarse kelp, 160 leached ashes, 30 pure ashes, 80 of brick clay, 

 about 100 broken glass ; or, soap maker's waste and river sand, in 

 proportions determined by practice. Common sand and lime, with 

 some common clay, and sea salt, form a good mixture for bottle glass. 



3. Pastes are artificial imitations of the gems. They are very 

 fine glass, rendered fusible by borax and other fluxes, and stained by 

 oxides of metals. Rock crystal, or other very pure siliceous mat- 

 ter, is selected, pulverized very fine, and mixed with the other sub- 

 stances ; the following examples will shew the composition. 



Pulverized rock crystal, or flint, 8 oz. purified pearl ashes, 24 oz. 

 these are fritted together, and then mixed with 12 oz. of white lead, 



* Consisting of refuse lime, that had been used to give causticity to the alkali, 

 the insoluble part of the kelp or barilla, and some salt and water, all in a pasty state. 

 Ure. N 



t lu England, the government will not permit any but coarse sand to be used in 

 this manufacture, lest (lie common glass should be so good that the sale of the flint 

 and other superior kinds of glass, which pay a higher duty, should be diminished. 

 Parkes. 



