Mike Shaw wrote:
>Date: Fri, 11 Jan 2002 12:54:54 -0000
>From: Michael Shaw <[log in to unmask]>
>Subject: Re: watercolour pigments
>J B Richardson's book 'Metal Mining' states that a watercolour pigment is
>made (or was then) from witherite, can any one tell me what this pigment was
>and if it is still produced.
>all the best.
Mike:
This may not be the ONLY answer, but in short Witherite, PbCO3, was used as an
adulterant in lead paint. The rather gruesome details are spelled out in the
quotations given below.
1). Dana's Textbook of Mineralogy (originally publ. 1898), 4th Ed., 20th
printing, 1963 (J.W. Wiley). by W.E.Ford, p.523:
"WITHERITE:
Orthorhombic....crystals always repeated twins..... also massive, columnar or
granular.
Fracture uneven,..Brittle, H=3-3.75. Luster vitreous, inclining to resinous on
fractures. Color white, yellowish, grayish. Streak white (this is important, as
it means that the ground powder would be white). Subtransparent to translucent.
COMP: Barium CARBONATE, BaCO3 = Carbon Dioxide 22.3, baryta 77.7
Soluble in HCl.
Obs. --Witherite is of infrequent occurrence. It is most commonly found in
veins assoc. w. galena. In England important occurrences are at Alston Moor in
Cumberland assoc. w. galena and in Northumberland at Fallowfield near Hexham. At
the latter locality it occurs in large quantities and often in splendid
crystals. In Japan.......
Use. -- A minor source of barium compounds."
Dana says nothing about any use as a pigment: from the above it would make a
good white pigment, but would be unsuitable for demanding uses because of its
softness, and its solubility in HCl.
2). However, Kraus, Hunt and Ramsdell's "Mineralogy", 5th ed., 1959
(McGraw-Hill), p.335, says that witherite is used to "adulterate white lead, in
the extracting of sugar from sugar beets, and in ceramics, glass and drilling
muds. "White lead" is lead carbonate (PbCO3), KH&R say that it is an important
ore of lead and silver, and occurs at Leadhills, Scotland. "
Neither they nor Dana mention any use as a pigment (presumably because they are
both geologically-oriented!).
3). Finally, Holmyard's "Revision Course in Chemistry" (my 'A" Level
swatting-up text), J.M. Dent, London, 1931, reprinted 1954, page 232 says:
"Lead Carbonate:
Basic lead carbonate, 2PbCO3).Pb(OH)2, is known as "white lead" (pigment, and
basis of oil-paints). It is precipitated by adding sodium carbonate to a
solution of a lead salt, but is manufactured by hanging lead sheets in pots
containing dilute acetic acid, the pots being packed round with horse-dung, or
other decaying organic matter".
The acetic acid converts the lead into a basic acetate of lead, and this reacts
with the carbon dioxide given off from the horse dung to form white lead. The
white lead is scraped off the lead sheets when the latter are nearly completely
destroyed. "
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