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Essential Fire Assay Terms



as used in Textbooks

"Surcharge" - "check" - "proof" - "standard"




SurchargeNet Losses and Gains — The net sum of the (...) losses and gains incurred in the various operations is called the "surcharge" since, in the assay of high-grade bullion, the silver retained is usually in excess of the gold lost, so that the cornet will weigh more than the gold originally present in the assay piece; if the reverse is the case, the result is considered by some assayers to be less accurate. When there is a gain in weight it is referred to as a "plus surcharge", while a loss in weight is designated a "minus surcharge".

With material rich in gold, the silver retained more than compensates for the loss, and, as just stated, the surcharge is positive; but with a decrease in the proportion of gold the loss is greater and the surcharge is negative.

Thus the surcharge will usually amount to about 0 (nil) for bullion of about 700 to 800 fine; above that there will be a "plus surcharge" and below that a "minus surcharge". The surcharge is reported in parts per 1000 in the same way as the fineness of bullion. (p. 276)


... certain losses and gains (the sum of which is called the surcharge) take place during the cupellation and parting of gold alloys, the losses being due to (1) absorption by the cupel, (2) volatilisation and (3) solution in the acid and the gains to the retention of silver by the gold cornet.

The amount of the surcharge varies with the conditions of working and the composition of the alloy, the general experience being that the losses under all three heads are greater as the percentage of copper in the alloy increases. (p. 328)


The total losses may or may not be counterbalanced by the silver retained by the gold after parting, which amounts to about 1 part per thousand under normal conditions of working.

In practice it is found that with alloys rich in gold the silver retained by the cornet more than compensates for the various losses and the surcharge is positive; but with alloys of low standard the losses are greater and the surcharge is negative, the cupellation loss being usually greater on account of the larger proportion of copper present, as previously pointed out. (p. 330)


Since the losses and gains are dependent on so many conditions, the surcharge is best determined by the use of "checks" or "proofs" consisting of pieces of pure gold which are subjected to the various operations side by side and under identical conditions with the assay pieces.

The composition of the checks is the same as the composition of the bullion being assayed in order to make the assays absolutely comparable. Being, therefore, of the same composition and weight and undergoing exactly the same treatment, the checks may reasonably be expected to have the same surcharge as the assays which they imitate. The checks are made up from the data obtained by a preliminary assay. (p. 276)


Consequently it is necessary to subject pieces of fine gold (called "proofs" or "checks") to the various operations side by side and under identical conditions with the assay pieces, and thus to determine the "surcharge" or net sum of the losses and gains incurred in the various operations.

The surcharge is usually positive, a proof weighing 1,000 at the beginning attaining a weight of about 1,000.5 at the end.


If there is a surcharge in the check of more than 1‰ or 2‰ in the first cupellation, showing retention of copper, the prills are re-cupelled with fresh lead.


In the bullion assay for gold, the algebraic sum of the errors outlined, the losses being designated minus and the gains plus, is called the "surcharge."

In the gold bullion assay this will vary from +0.025 %, in very pure gold bullion, to 0.25 %, in base bullion, passing to zero for a bullion about 800 fine. (p. 173)


This surcharge will usually amount to about "0" for a bullion of about 700 to 800 fine; above that there will be a "plus surcharge," and below that a "minus surcharge." The plus surcharge will be subtracted and the minus surcharge added. (p. 185)


The proofs always show a slight gain in weight. The correction thus determined is termed the "surcharge," and is really the algebraic sum of all the gains and losses. (pag. 231).


The sum of the errors of an assay, which is called the surcharge, is reported in the same way. Thus a surcharge of + 0.3 means that the gold as weighed was 0.3 parts per 1000 more than the gold actually present. But a surcharge – 0.3 means that on the whole there was a loss of 0.3‰ in the assay.

Speaking roughly the retained silver will vary with the weight of gold present; if one alloy contains twice as much gold as another the retained silver will be about twice as much also.

On the other hand, as already explained, the cupellation loss on the poorer alloy is as much as, or even more than, with the richer one, because of the copper, &c. present. With rich gold alloys the silver more than compensates for the loss and the surcharge is positive; but with poorer alloys the loss is greater and the surcharge is negative.

















proof or check (assay) / standard



témoin /


testigo /



Zuschlag /












Fire Assay




 free download from

► Internet Archive

► I. A.

"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

Terms: ►Cupellation  ►Fire Assay  ►Reagents  ►Other Methods  ►Metals Sheets: ►Cupels  ►Crucibles  Index: ►Programme