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Essential Fire Assay Terms



as used in Textbooks

"Spit" - "Spitting" - "Sprout" - "Sprouting"  - "Spurting"-  "Vegetation"

at end of cupellation




Molten silver absorbs about twenty-two volumes of oxygen from the air, and the gas is given off again during solidification, usually with much vigour. The solidified crust of silver is generally broken and blistered, and particles of metal are projected by the gas disengaged from the interior (the so-called "spitting" or "sprouting").


If no copper had been added, the disengagement of oxygen at the moment of solidification would cause the button to spurt or "spit," and the assay would be spoilt by the loss of some metal.


... the furnace is allowed to cool down slowly and evenly. If the molten beads were removed before solidification, spitting would result, but as soon as they have solidified they are withdrawn from the furnace, ...


Platinum is not affected by heating in dry or moist air. When molten it absorbs oxygen which is given off on cooling, sometimes with sufficient rapidity to cause the metal to spit like silver.


Liquid silver has the peculiarity of dissolving a large volume of oxygen, which is expelled upon solidification. The solidification of large beads starts at the surface. When the center of silver beads solidifies, the oxygen is sometimes expelled violently, spewing forth some of the interior silver and forming a cauliflower-like growth on the bead. This mechanism is known as "sprouting."

Sprouted beads should be rejected if there is any reason to believe that, any particles of metal have been lost.

To avoid sprouting, all large beads should be cooled slowly, either by leaving them in the, furnace after the blick, and until certain that, they have cooled sufficiently, or by moving them to a slightly cooler zone in the furnace and then covering them with a very hot cupel before they have completely solidified.

The hot cupel melts the outer crust and allows the bead to solidify slowly, so that the oxygen can escape from the interior without violence. When the bead contains more than one-third of its weight in gold, it does not sprout; hence beads known to be of this composition may be removed from the furnace as rapidly as desired.

Sprouting of silver beads is considered a sign of purity, but this indication is of no practical value to the assayer, as sprouting should always be avoided. (p. 63)


Bugbee states that as little as 0.004% of rhodium in silver beads causes a distinct crystallization, which is more apparent at 0.01% Rh, and that 0.03% Rh causes unavoidable sprouting of silver beads. (p. 71)


... there is no danger of sprouting if the silver-gold ratio is less than 3 to 1, and if copper is present. (p. 182)


Sprouting, however, is considered proof of the absence of all but traces of impurities. (pag. 114)


Instead of attempting to prevent sprouting by covering with a hot cupel, the student may try the following little-known method, first described by Aaron. After brightening, the cupel is drawn to the front of the muffle and gently tapped on one side with the tongs.

At the instant when the bead ceases to vibrate in response to the taps, by which is indicated the beginning of solidification, it is pushed back into the hottest part of the muffle and left for about a minute. After this it may be entirely withdrawn and will not sprout, being solid all through, as shown by a "dimple" in its surface, caused by contraction. (p. 228)


Silver beads after cupellation, and at the moment of solidification, also "sprout".

According to Gay-Lussac molten silver dissolves 22 times its volume of oxygen, at the freezing-point. Later researches prove this practically correct. At 1020º C molten silver will hold 19.5 volumes of oxygen (at 760 mm and 0º C) and at the melting-point somewhat more. For any given temperature the oxygen dissolved is proportional to the square root of the oxygen pressure. In air at 760 mm pressure the oxygen has a partial pressure of 150 mm and the volume of oxygen dissolved by molten silver under assay conditions is 9.65 volumes at the freezing-point of silver.

The oxygen is dissolved either as monatomic oxygen or as silver oxide (Ag2O), in dilute solution. It is probable that this silver oxide, not being soluble in solid silver is dissociated with explosive violence, with the liberation of oxygen, when the silver solidifies.

This oxygen, suddenly expelled when the bead solidifies, causes a cauliflower-like growth on the bead. Small particles of silver may even be projected from it and cause a serious loss. When gold is present in the silver bead to the extent of 33 %, or more, sprouting does not take place. Silver beads containing small quantities of Pb, Cu, Zn, Bi, etc., will not sprout, so that if a button does sprout it is a sign of purity.

Buttons below 5 mg in weight do not sprout readily; large buttons, however, do. Sprouting can be prevented by slow cooling in the muffle, or by having ready a hot cupel which can be set, inverted, over the one holding the bead, and withdrawing both from the muffle, thus cooling the bead slowly. Sprouted beads are to be rejected as an assay. (p. 83)


Molten silver dissolves oxygen from the air and gives it off on solidifying; the escape of the gas on sudden cooling is violent and, by throwing off particles of the metal, may cause loss. This is called "vegetation" or "spirting."

The silver is apparently solid when spirting takes place; the crust breaks suddenly and some of the metal is forced out. The evil is best guarded against by slow cooling and avoiding draughts.

With large buttons of silver precautions should never be omitted. One plan is to allow the cupels to cool in the muffle itself, the mouth being closed with hot charcoal. Another is to cover the cupel with another cupel previously heated to redness; in this case the silver cools between two hot cupels, and, of course, cools slowly. A third plan is to withdraw the cupel to the door of the muffle, holding it until it begins to get solid and then immediately to put it back into the hotter part of the muffle.


But the danger of spirting decreases as the proportion of gold becomes greater, and disappears when the gold is much over 30%.

Nevertheless it is well to let such buttons become solid undisturbed and protected from draughts in the body of the muffle. This means closing the muffle and allowing the furnace to cool down somewhat before withdrawing the cupels.


Silver when molten absorbs oxygen from the air and gives it off suddenly when solidifying; causing a cauliflower-like growth on the surface of the button and particles of silver may even be ejected out of the cupel and cause serious loss. This "spitting", "spurting" or "vegetation" as it is termed does not take place readily with silver buttons weighing less than about 5 mg, but all buttons above this weight are very liable to spit. Spitting does not take place if gold is present in the silver button to the extent of 33 % or more (Levol), as the solubility of oxygen in silver is lowered by alloying with gold. If the button is small the cupel may be withdrawn from the muffle as soon as cupellation is finished without risk of "spitting", but with large buttons precautions must be taken.

Spitting can be prevented by slow cooling and in practice the following methods are usually adopted. The cupels are allowed to cool in the muffle itself, the door being closed; or another cupel, previously heated to redness, is inverted over the cupel containing the button and both carefully withdrawn from the muffle, thus cooling the button slowly between two hot cupels; or the cupel may be withdrawn very gradually towards the door of the muffle.

Cooling in the muffle is invariably adopted with a large batch of assays, as, in addition to the danger of upsetting a cupel, it is inconvenient to handle a large number of hot cupels.

Buttons that have "spitted" must be rejected and the assays repeated. (p. 160)


The oxygen evolved often bursts through the outer crust of solidified metal with considerable violence, ejecting portions of the still liquid silver as irregular excrescences constituting the phenomenon known as the "spurting", "spitting" or vegetation of silver. This may be prevented by sprinkling charcoal powder on the melted metal. The presence of small quantities of copper also prevents the spurting of silver. Silver alloyed with as much as one-third of its weight of gold still retains the power of absorbing oxygen and spurting on solidification, but larger proportions of gold prevent the action (Percy). (p. 71).


The cupels retain the heat longer than the muffle and if the cooling is too rapid, the buttons will solidify on the exposed surface while the interior is kept molten by contact with the hot cupel and when the button solidifies, spitting is very liable to take place.

If, however, the cupel is slowly cooled from below, the under surface of the button will solidify first and the dissolved oxygen escape before the silver solidifies as a whole. (p. 184).


The copper is added as the small quantity that remains in the button tends to prevent "spitting" or vegetating after cupellation and also to increase the malleability of the button for rolling. (p. 267)


To avoid losses by "spitting", etc., most assayers allow the assays to remain in the furnace until the buttons have solidified. To effect this the muffle-door is closed and the temperature lowered. The buttons when "set" (solidified) should show a depression on the top and no "vegetation". (p.268)

















sprouting, vegetation

sprießen, Sproß, Blumenkohl

spruitje /



arborescencia /








Fire Assay




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► 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

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