306 
Chromat of Lead, 
of lead, occasioned all the varieties in shades, in which the pig- 
ment was exhibited in our experiments. Under that impression, 
I digested a portion of the precipitate which I had obtained in for- 
iner experiments, in cold diluted nitric acid, a slight effervescence 
ensued ; and the colour immediately improved to a lively orange 
yellow : nearly equal to that which you think superior to any you 
have seen prepared. 
The result of the above experiment suggested the following 
processes, The first I have verbally communicated to several per- 
sons, who have undertaken to prepare the pigment for sale : but, 
from the inferior colour which they produce, I fear that more 
knowledge of chymical processes is necessary to enable a person 
to conduct the second than is common to mere manufacturers. 
1 st. Reduce any quantity of chromat of iron to a powder suffici- 
ently fine to pass through a sieve of the finest bolting cloth ; tritu- 
rate half its weight of nitrat of potash, and mix the two powders inti- 
mately together. Introduce the whole into an iron pot (I use the pot 
of a cast iron glue-kettle, having a cover) and add as much water 
as will reduce the whole to the consistence of thick paste. Place 
the pot on some lively coals, and cause the water to boil, stirring 
the whole occasionally with a stick, until the mass is dry; then in- 
troduce the pot into an air furnace, and heat it about a cherry red ; 
but be careful not to increase the temperature as long as oxygen 
gas is evolved, otherwise the pot will be melted. The escape of 
oxygen gas will be made manifest by holding a live coal at the 
juncture of the pot and cover ; as long as oxygen escapes, the coal 
will burn with increased splendor ; when the splendid combustion 
ceases, the fire should be augmented until the pot is of a very 
bright red, and kept at that temperature about one hour. The 
pot should now be suffered to cool in the furnace, to prevent its 
cracking by a sudden change of temperature ; and when cold, it is 
to be well wiped on the outside, and placed in a tinned kettle con- 
taining a large quantity of water, which is to be boiled until the 
mass can be detached from the pot ; when the latter maybe with- 
drawn. The boiling must now be continued for half an hour, oc- 
casionally bruising the hard mass at the bottom of the kettle, after 
which it may be withdrawn from the fire and suffered to settle for 
a few minutes. When the supernatant liquid is clear, it is to be 
decanted into a convenient vessel, and the kettle filled with fresh 
water, which must be boiled on the residuum, and the process re- 
peated as often as the liquid appears to be strongly coloured. Af- 
ter a few boilings, the liquid (if the quantity of water is large) will 
