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Essential Fire Assay Terms

13-Mar-17

TContext04_Feathers.doc

as used in Textbooks

"Feathers"

Author

Context

Fulton

... crystals of litharge (feathers) form on the side of the cupel toward the muffle mouth. If the temperature is too low for the cupel to successfully absorb practically all of the PbO, these feathers form low down in the cupel. When the temperature is about right, they form near the upper rim of the cupel. It is, however, to be noted that the draft through the muffle influences the formation of feather litharge; i.e., if the draft is strong, feathers will form, although the temperature is somewhat above 820º C. (p. 82)

Shepard

.. feathers," will appear around the upper edge of a bone-ash cupel. This is due to the condensation of that small part of the litharge which is volatilized at the surface of the lead. If the temperature is too high the volatilized litharge is carried away in the furnace gases or is deposited on cooler projecting surfaces in the path of the air stream as it passes through the furnace.

If the air draft is too strong, feathers may appear only on the side of the cupel toward the draft, or may not form.

As cupellation proceeds and the lead button becomes smaller, concentric rings of feathers are deposited within the. original ring, but at all times the temperature should be high enough so that there is a clear area between the button and the feathers, otherwise there is danger that a pool of litharge will form immediately around the button, which will instantly prevent further absorption of litharge by the cupel, and the litharge will soon completely cover the lead and solidify, which will cause the button to freeze.

Even though the assayer fails to note the encroachment of feathers toward the button, or the decrease in color temperature of the lead, the onset of freezing is plainly evident in the oily appearance of the ring of molten litharge at the outer periphery of the button, and the temperature should be raised immediately to avoid freezing.

Individual cupels that show signs of incipient freezing may be saved by placing a hot cupel or brick near or over them. (p. 59)

Shepard

The formation of feathers of litharge can be observed readily with bone-ash or bone-ash-cement cupels and serves to indicate proper driving temperature, but when copious
feathers form on magnesia cupels the temperature is dangerously near the freezing point. (p. 66)

Smith

"Feathers" are crystals of solid litharge sublimed from the vapour and deposited on the rim of the cupel, which is invariably cooler than the molten lead on the cupel. It is stated by Fulton that they will not form above 820º C. (p. 162)

Smith

When cupellation of silver is carried on under these conditions the temperature should not, according to Fulton, be above 820° C, in which case crystals of litharge (feathers) form on the side of the cupel towards the muffle mouth.

If the temperature is too low for the cupel successfully to absorb practically all the litharge, these feathers form low down in the cupel.

When the temperature is correct, they form near the upper rim of the cupel. It is, however, to be noted that the draught through the muffle influences the formation of feather litharge; i.e. if the draught is strong, feathers will form, although the temperature is somewhat above 820º C. (162)

Smith

The formation of "feathers" is generally accompanied by a sluggish, heavy movement of the fumes, which fall in the muffle.

Beyond the fact that it shows that the muffle has not been too hot, it is doubtful whether there is any advantage in cupelling at a temperature sufficiently low to permit of the formation of feathers. (p. 163)

 

 

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feathers (litharge)

Feder (Bleioxid)

veer (litharge)

plumes (litharge)

penne (litargirio)

plumas (litargirio)

перья (окись свинца)

 

 

 

 

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REFERENCE BOOKS
"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


Bugbee
Shepard
Fulton
Smith
Rose1
Rose2
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