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Essential Fire Assay Terms



as used in Textbooks

"Matte" ("regulus") - "Speiss"




MATTE is an artificial sulphide of one or more of the metals, formed in the dry way. In assaying it is most often encountered in the niter fusion of sulphide ores when the charge is too acid. It is found lying just above the lead button. It is usually blue-gray in color, approaching galena in composition and very brittle. It may form a layer of considerable thickness, or may appear simply as a granular coating on the upper surface of the lead button. This matte always carries some of the gold and silver and, as it is brittle, it is usually broken off and lost in the slag in the cleaning of the lead button. The student should examine the lead button as soon as it is broken from the slag and if any matte is found, he may be certain that his charge or furnace manipulations are wrong.

SPEISS is an artificial, metallic arsenide or antimonide formed in smelting operations. As obtained in the fire assay, it is usually an arsenide of iron approaching the composition of Fe5As. Occasionally the iron may be replaced by nickel or cobalt. The antimony speiss is very rare. In assaying, speiss is obtained when the iron method is used on ores containing arsenic. It is a hard, fairly tough, tin-white substance found directly on top of the lead and usually adhering tenaciously to it.

If only a small amount of arsenic is present in the ore, the speiss will appear as a little button lying on top of the lead; if much arsenic is present, the speiss will form a layer entirely covering the lead. It carries some gold and silver. If only a gram or so in weight, it may be put into the cupel with the lead and will be oxidized there, giving up its precious metal values to the lead bath. A large amount of speiss is very hard to deal with as it is difficult to scorify. The best way is to assay again, by some other method. (p. 15)


Slags for Pure Ores. – When an ore contains so large a proportion of sulphide minerals that it is necessary to add niter to prevent the reduction of too much lead, it will be found that the charges recommended for Class 1 ores will not allow a satisfactory decomposition of the ore. Instead of two products, slag and lead, a third intermediate product, matte, is often obtained as the result of the fusion. This amounts to an incomplete decomposition of the ore and as matte is a good collector of precious metals its presence is a sure indication of low results.

A matte is much less likely to be formed, however, with a less acid charge and it has been found best, therefore, to make a slag approaching a monosilicate for all sulphide ores, as by this means more uniformly satisfactory results are obtained. (p. 175)


Phases. — In the melts used in assaying, smelting, and fire refining there is the possibility of producing a number of separate liquids that are not miscible with each other and, therefore, segregate into separate layers called "phases". Any or all of the phases — metal, speiss, matte, slag, and molten alkaline salts — may be formed. The metal phase has the highest density and forms the bottom layer; the other phases separate above the metal phase according to their densities, which are usually in the order given above.        ......

The speiss and matte phases are avoided in fire assaying by proper conduct of the assay. Speiss consists of the arsenides and antimonides of iron, cobalt, nickel, or copper; matte is a mixture of fused sulfides, usually of iron and copper. Speiss is heavier than matte but lighter than lead; hence it forms a layer between the lead and the matte, if all three are present.

The slag phase, consisting of metal oxides and silica or borax glass, lies above the matte layer or, if matte and speiss are absent — as should be the case in an assay fusion — the slag rests directly upon the lead.      (p. 86)


Whether a metal appears in the metal layer or in the slag depends upon whether or not it is combined with oxygen. The metal oxides are quite soluble in the slag and, in general, are insoluble in the metal. Consequently, when two different metals are present, if one can be oxidized to metal oxide it will go into the slag layer, while if at the same time the other metal can be prevented from oxidizing it will appear in the metal layer, provided no speiss or matte is formed. This principle forms the basis of the separations made in fire assaying, smelting, and fire refining. (p. 88)


In the presence of copper, speiss will form in the assay fusion unless the charge is proportioned with sufficient excess of litharge and soda to keep the copper as well as the arsenic and antimony oxidized.

Uncontrolled reduction methods should never be used for assaying materials containing large amounts of arsenic or antimony. The excess reduction used in uncontrolled reduction methods will reduce arsenic and antimony, as well as some of the speiss-forming metals, so that speiss is almost certain to form.

When a speiss phase forms, it is found as a brittle substance just above the lead button. It can be distinguished from matte by its bright metallic luster.

The danger of loss of precious metals in the presence of speiss occurs primarily through loss of small fragments of speiss when separating the lead button from the slag; but even though
all the
speiss could be recovered, it cannot be cupeled satisfactorily  ....

Artificial sulfides of the heavy metals are called "matte." In general, mattes are almost completely insoluble in slag and have only slight solubility in metal or speiss phases, but they are soluble in molten alkaline salts such as sodium carbonate and, particularly, sodium sulfide. (p. 111-112)



In addition to slag and metallic lead, other products may be obtained as the result of the fusion of an ore according to the constituents present. If metallic sulphides are present, an artificial sulphide or regulus (sometimes termed matte) may be formed and if the ore contains arsenical minerals, a compound of a metal or metals with arsenic, termed a speiss, may result.

Assuming these substances to be present in a charge, they would separate according to their densities, when the fused mass solidified, in the order shown in fig. 100A.

At the bottom of the crucible a button of lead would be found; above this a thin layer of speiss; then a regulus, next a slag, and, in special
cases, on the top of this a layer of more fluid
slag consisting usually of fusible alkaline chlorides and sulphates.

A regulus (also termed matte), which is a compound of one or more metals with sulphur, is a yellowish-grey, metallic-looking mass, usually brittle and often crystalline. A speiss is usually hard, brittle and greyish-white in appearance. (p. 127-128))

















matte, regulus

Matte, Stein, Lech





















Fire Assay




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"A Textbook on Fire Assay"                                                                Edward E. Bugbee
"Fire Assaying"                                            Orson C. Shepard & Waldemar F. Dietrich
"A Manual on Fire Assaying"                                                      Charles Herman Fulton
"The Sampling and Assay of the Precious Metals"                           Ernest Alfred Smith
"Metallurgy of Gold"                                                                                Thomas K. Rose
"The Precious Metals
: comprising Gold, Silver and Platinum"               Thomas K. Rose
"A Text Book of Assaying"                                                   C. Beringer & J. J. Beringer







Context: ►Assay Ton  ►Blank  ►Steps   ►Feathers  ►Matte/Speiss  ►Colours  ►Spitting  ►Sprout  ►Surcharge  ►Inquart  Hallmark  ►Cupels


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