First Assay

160. 1st Assay. Half an ounce of the finely pulverized and dried ore is roasted in a roasting dish, in the muffle. To prevent loss by decrepitation, the assay is covered by another roasting dish inverted. A small hole should be bored through the cover, and a few notches broken in its edge, so that the arrangement resembles a small reverberatory furnace.

The heat is kept very low, and after a time the dish is taken from the muffle and cooled a little, to prevent decrepitation, and consequent loss, on uncovering it. The cover is then removed, and the ore well stirred and turned over, again covered, and returned to the muffle. This is repeated until it is found that the assay may be uncovered and stirred within the muffle, without danger of loss by decrepitation. The heat is then increased to bright redness, and so maintained until the ore is odorless. After cooling, the assay is dressed thus:

Ore (roasted)...................

Litharge...................... .2 to 4 ounces

Sodium bicarbonate..............1/2 "

Flour..........................40 grains.

Mix, and place in a No. 8 French crucible, containing 1/2 ounce of borax in crystals, and cover with salt. Fuse quickly in a strong fire. Allow the assay to remain five minutes in the fire after fusion, and pour. The lead button should weigh about 300 grains. If it varies much from that weight, the assay should be repeated, with a larger or smaller quantity of flour as the case may be. A duplicate assay should be made, with either more or less litharge, as a check. The lead buttons are cupelled as usual.

161. 2d Assay:Ore (not roasted)................1/2 ounce

Litharge.......................2 to 4 "

Sodium bicarb ...................1/2 "

Dried borax....................1/2 "

Pulverized nitre...................420 grains.

Mix; put into a No. 8 French crucible, and cover with salt. Heat the assay very slowly until fused; raise the heat to bright redness, and keep it so for five minutes at least; then pour.

The button should weigh about 300 grains, and be soft. No matte should be formed. If these conditions are not fulfilled the assay is imperfect, and must be repeated with a different proportion of nitre, more if the button is too large, or if a matte is formed; less if it is too small. Theoretically, 25 parts of nitre convert 100 parts of lead into litharge; in practice the proportion varies with the purity of the nitre.

162. 3d Assay.Ore............................1/4 ounce

Litharge.......................2 to 4 "

Sodium bicarb...................1/2 "

Dried borax.....................1/2 "

Pulverized nitre..................300 grains.

Place the mixture in a No. 8 crucible; cover with salt; fuse very slowly. When ebullition ceases, under a good red heat, add 20 grains of charcoal in lumps, a portion at a time. When the evolution of gas produced by the charcoal ceases, cover, and heat strongly for ten minutes; then seize the crucible with the tongs and shake it, without removal from the fire. Again cover, and leave it in the fire for a few minutes longer, and pour.

In this assay, the quantity of nitre is sufficient for the complete oxidation of even pure iron pyrites. Any excess is decomposed by the heat, and after its decomposition the addition of 20 grains of carbon generally reduces a suitable quantity of lead, which, for 120 grains of ore, is about 150 grains. If it does not, the carbon alone is varied. The method is a little tedious, but gives very good results, especially as to gold.

163. In all the preceding assays the crucible is liable to extensive corrosion, unless the ore contains a considerable quantity of quartz. This may be prevented by the addition of half or a whole teaspoonful of pulverized quartz, which must of course be free from precious metal. Glass will answer the purpose, but rather more of it is required.

164. An assay which is dressed with undried borax is less liable to boil over if put at once into an intense fire, than if heated by slow degrees. When nitre is used rapid heating is not admissible, hence the borax is dried before use. It is well to dry it in all cases. Boiling over sometimes occurs in consequence of an evolution of carbonic acid gas (carbon dioxide) produced by the fusion of the alkaline carbonate, used as a flux, with silica. A large proportion of litharge, and less of the carbonate, will correct this, while forming an equally good slag. Whenever the assay tends to boil over, it may be cooled down by throwing a little dry salt into it.

165. 4th Assay.Ore......................... 120 grains.

Litharge.....................6000 "

Cover with salt; fuse quickly, and pour as soon as the assay becomes quiet. The lead button should weigh about two ounces, if the ore consists chiefly of iron pyrites. It must be reduced by scorification to a suitable size for the cupel.