890 Researches respecting the 
ceasing to sparkle ; that is to say, when yon no longer see 
any parts burn like coal, and when the whole appears of 
an uniform colour. When the proportion of tin exceeds 
25 or 80, a stronger fire is necessary to produce the calci- 
nation. In a word, by varying the degrees of heat you 
will be able to discover that best suited to the mixture on 
which you operate. 
A hundred parts of the calx above mentioned, which in 
the French potteries is called calcine , is generally taken 
with 100 parts of sand. From 25 to 80 pounds of sea- 
salt, or muriat of soda, are added : the whole is well 
mixed together, and it is fused in the bottom of a furnace 
In which potter’s ware is baked. This matter is gene- 
rally placed on sand, on lime quenched in the open air, 
or on ashes. The bottom of the mass is in general badly 
fused. This, however, does not prevent the matter, after 
it has been pounded, and applied on the articles, from be- 
coming exceedingly white and hard in the furnace. 
When taken from the furnace it is not white; it is even 
often very black : in general it is marbled with black, 
gray, and white. 
This process is that generally used in potteries. In 
the compositions destined for earthen-ware, the propor- 
tion of 25 parts of tin to 100 of lead is never exceeded: 
for common earthen-ware, the manufacturers are even sa- 
tisfied with 15 of tin to 100 of lead. It may be easily 
seen, that if you wish to obtain an enamel whiter and 
more fusible, you must dimmish the quantity of sand; but 
there is no necessity for augmenting that of the sea- salt, 
or muriat of soda: as the whiteness and opacity depend 
on the quantity of tin, you may use calcine , which con- 
tains 25 or 80 per cent. For example, 100 of such cal- 
cine, 60 of sand, and 25 of marine salt, give a composi- 
tion exceedingly fusible.- 
