CLASS II.

Assay of Substances of the Second Class.-The assay of these substances is very simple indeed. Litharge, minium, carbonate of lead, &c., may be assayed by simple fusion with carbonaceous matter: but when the operation is thus conducted loss of lead is sustained: it is therefore better to add some flux which will readily fuse, and allow the globules of reduced lead to collect into one button. No flux fulfils this condition better than a mixture of carbonate of soda and argol, which is to be intimately mixed with the assay. The following is the best mode of procedure:-To 200 grains of the finely-pulverised substance add 100 grains of argol, and 300 of carbonate of soda, and intimately mix; place the mixture in a crucible which it about half fills, and cover with a layer of common salt about inch thick; submit the crucible to a very gradually increasing temperature, keeping the heat at low redness for about a quarter of an hour; then urging it to bright red until the contents of the crucible flow freely; take it from the fire and shake, tap it as directed in the copper assay, and either pour the contents into the mould or allow to cool in the crucible. If the operator be pressed for time, the mould may be used, but it is recommended to allow the assay to cool in the crucible, for unless the operator be very careful, and have had some considerable practice, he is very liable to lose a small quantity of metal in the pouring. After the contents of the mould or crucible, as the case may be, are cold, the lead may he separated from the slag by repeated gentle blows from the hammer: if any of the slag or crucible adhere to the button, the latter may be readily freed from it by placing the button between the finger and thumb with its edge on the anvil, and then gently hammering it. The lead will be so altered in shape under the hammer that the slag or crucible readily falls off; and by continuing the process, whole may be removed. The cleaned button may then be hammered into a cubical form, and is ready for weighing.

the

In the assay of lead great care must be taken in the

management of the temperature, as lead is sensibly volatile above a bright red heat, even when covered with flux, and still more so if any portion be uncovered from want of sufficient quantity of flux; neither must the assay remain in after the flux flows freely, for a loss may thereby occur from oxidation, by decomposition of carbonate of soda, as explained in the reduction of copper ores and the copperrefining process.

For the rationale of this mode of assay, refer to page 192, which explains the decomposition of oxide of lead, with the production of metallic lead, carbonic acid, and water, by the agency of a substance, like argol, containing both carbon and hydrogen.

Cupel bottoms, some lead fumes, and siliceous slags, require a modified treatment in their assay, as the substances mixed with the oxide of lead (more particularly bone-ash in the cupel bottoms) are very infusible; and if the flux already mentioned as applicable to the other matters belonging to this class were employed, a very high temperature would be necessary; and as lead, as already stated, is sensibly volatile above a bright red heat, an evident loss of that metal would be the result.

Cupel bottoms may be thus assayed: 400 grains of the finely-pulverised bottoms to be mixed with 200 grains of argol, 400 grains of carbonate of soda, and 400 grains of pulverised fused borax; the mixture placed in a crucible as already directed, covered with salt, and the fusion conducted as just described.

Lead fumes and siliceous slags require only half their weight of fused borax, with 200 argol, 400 carbonate of soda, and 400 substance (fume or slag) covered with salt.

The addition of the borax, which is a most powerful flux, causes the fusion of the assay to take place almost as readily with the last-named intractable substances, as with the former easily fusible and reducible matters. The assay, however, is rather more subject to ebullition or boiling over the sides of the crucible; hence it must be carefully watched, and the instant it appears likely to do so the crucible must be removed from the fire, gently tapped on the furnace top,

and when the effervescence has subsided returned to the furnace, and this operation repeated until the fusion proceeds tranquilly.

The lead obtained in these assays, if the ore or substance contained any foreign metal, is never pure: if silver, copper, tin, or antimony be present, the whole of either of these metals will be found alloyed with the lead produced; but if the ore contains zinc, and it be heated sufficiently, but a trace remains; nevertheless the zinc carries off with it a considerable quantity of lead.

The following experiments will show what an influence the presence of zinc has upon the return of lead :100 parts of litharge,

100 parts of oxide of zinc,
300 parts of black flux,

were fused together, and 84 parts of lead were the result. 100 parts of litharge,

100 parts of oxide of zinc,

600 parts of black flux,

were fused together, and but 70 parts of lead were produced, instead of 90, which the pure litharge ought to have given. Hence it will be seen that, the more zinc is reduced, the more lead is volatilised.

If oxide of iron be present in the assay, it is reduced, but it remains in suspension in the slag, and the lead does not contain a trace when it has not been too strongly heated. If the assay be made at a very high temperature, the iron may be fused, and then the lead will be ferruginous; this may be ascertained by means of the magnet. A similar result was obtained by many assayers, who thought for a long time that lead and iron could thus combine together; but by careful examination it is easily ascertained that the ferruginous buttons are but mechanical mixtures of lead and iron in grains. Indeed, by careful hammering, nearly all the iron may be removed from the lead, so that it loses its magnetic properties.

The oxides of maganese, when mixed with the ore, are changed into protoxide, which remains in the flux, and is not reduced.

Humid Assay of Ores of the Second Class.-Pulverise the substance very finely, and to 100 grains placed in a flask add one ounce of nitric acid diluted with two ounces of water (if minium be the substance to be analysed, it must be first heated to redness, so as to reduce the whole of the lead it contains to the state of protoxide), and gently heat, gradually raising the temperature to the boiling-point: when all action seems to have ceased, pour the contents of the flask into an evaporating basin, and evaporate to dryness with the precautions directed in the analysis of iron ore Allow the dry mass to cool, and a little dilute nitric acid, gently warm for an hour, then add water, boil, and filter. The whole of the lead now exists in the solution as nitrate: thus, say carbonate of lead had been the substance under analysis, then

PbO,CO2+NO,=PbO,NO,+CO,

To the filtered solution containing the nitrate as above, add solution of sulphate of soda, or dilute sulphuric acid, until no further precipitation takes place insoluble sulphate of lead will now be thrown down: this must be allowed to completely subside by standing in a warm place; and when the supernatant liquid is quite bright the sulphate may be collected on a filter, washed, dried in the water-bath, and weighed. It contains 68.28 per cent. of metallic lead.

The decomposition of the nitrate of lead by sulphate of soda may be thus expressed

Pb+O,NO + NaO,SO, = PbO,SO,NaO,NCO,. Determination of lead by standard solution will be described at the end of this chapter.

CLASS III.

Assay of Substances of the Third Class.-In the assay of bodies belonging to this class, a reducing agent must be employed but if that alone be used, the sulphates and arseniates produce sulphides and arsenides, and not pure lead. The action of another reagent is therefore necessary,

:

in order to deprive the lead of sulphur and arsenic with which it is combined.

There are two reagents known for the sulphates—they are the alkaline carbonates or metallic iron; but for the arseniates and arsenites iron must be employed, because the alkaline carbonates have no action on the arsenides.

In all cases black flux is employed: this furnishes a reducing agent for the oxides, and a flux for the earthy matters. Iron is added when the arsenites or arseniates are assayed; but that metal may either be employed or not, when the sulphates are operated upon. It is, however, always better to use it.

When a mixture of black flux and iron is employed, the assay is made in exactly the same manner as that of the sulphides (large nails are preferable whenever the use of iron is indicated in a lead assay). With the sulphate, the sulphide of iron formed combines in the slag with the alkaline sulphide; but it is not so with the arseniates and arsenites. The arsenide produced mixes neither with the lead nor the slag, but gives rise to the formation of a brittle matter which adheres slightly to the button of lead.

When only black flux is employed, either of the two following processes may be adopted :-First, the ore can be fused with four parts of common black flux; then, as in the case of sulphide, the excess of carbon determines the formation of a large quantity of an alkaline sulphide; and consequently produces a desulphuration of the lead. Secondly, it may be fused with such a proportion of black flux, containing only the requisite proportion of carbon to reduce the oxide of lead, or with an equivalent mixture of carbonate of soda and charcoal. Pure sulphate of lead fused with one part of carbonate of soda and four per cent. of charcoal gives 66 of lead; but in order to employ this method the richness of the ore must be known, and the dry way is then useless, excepting for the estimation of the silver these substances always contain.

Humid Assay of Substances of the Third Class.-These are treated in precisely the same manner as those of the preceding class.