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Re: A question (Talcum Powder)



A definition of Talcum Powder from
<http://mineral.galleries.com/Minerals/Silicate/TALC/TALC.htm> is below:

THE MINERAL TALC

Chemistry: Mg3Si4O10(OH)2, Magnesium Silicate Hydroxide
Class: Silicates Subclass: phyllosilicates
Group: Clays and also The Montmorillonite/Smectite Group.
Uses: an ornamental and heat, acid and electrically-resistant stone
(soapstone) used as counter tops, electrical switchboards, carvings, etc,
used as an ingredient in paints, rubber, roofing materials, ceramics and
insecticides. Most commonly known as the primary ingredient in talcum powder.

Talc is an important industrial mineral. Its resistance to heat,
electricity and acids make it an ideal surface for lab counter tops and
electrical switchboards. It is also an important filler material for
paints, rubber and insecticides. Even with all these uses, most people only
know talc as the primary ingredient in talcum powder. Mineral specimens are
not very common as it does not form very large crystals. However, it often
replaces other minerals on an atom by atom basis and forms what are called
pseudomorphs (false shape). The talc takes the form of the mineral it
replaces. A specimen of what looks like milky quartz is quite a suprise
when it not only has a soapy feel but can be scratched by a fingernail.

PHYSICAL CHARACTERISTICS:

Color is green, gray and white to almost silver.
Luster is dull to pearly or greasy.
Transparency crystals are translucent and masses are opaque.
Crystal System is monoclinic; 2/m. Crystal Habits: never in large
individual crystals, but if found are flattened tabular crystals with a
hexagonal cross-section, usually talc is found in compact or lamellar
masses. Forms pseudomorphs (false shape) of other crystals such as quartz,
pyroxenes, olivine and amphiboles.
Cleavage is perfect in one direction, basal.
Fracture is uneven to lamellar.
Hardness is 1 (can leave mark on paper)
Specific Gravity is 2.7 - 2.8 (average)
Streak is white.
Other Characteristics: cleavage flakes are slightly flexible but not
elastic and talc has a soapy feel to the touch.
Associated Minerals include serpentine, dolomite, magnesite, quartz,
pyroxenes, olivine, biotite and amphiboles.
Notable Occurances: include many mines up and down the Appalachian
Mountains and in California and Texas, USA; Germany; Florence, Italy;
Tyrol, Austria; Transvaal, South Africa and Shetland, Scotland.
Best Field Indicators softness, color, soapy feel, luster and cleavage.

It is also used in many cosmetic and industrial applications, grease
absorption one of them, but in printmaking to help with the adhesion of the
gum etch to the edges of the drawing. It also seems to be used by sprinking
to keep tacky things from sticking together.

Perhaps the user thought the pages were sticking, or perhaps that since it
looked like chalk powder (Calcium Carbonate) that it would help deacidify
the pages... Well I know more about talcum than I did before.

Peter

>While surveying a collection of books for preservation evaluation purposes =
>I came across a volume that had powder sprinkled throughout.  Included was =
>a note on a slip of paper that indicated the powder was talcum.  What =
>might have been the reason for using the talcum?  What is the best method =
>for removing it?  I ask because after brushing the powder away very =
>carefully there seemed to be a residue left behind on the page.
>
>- Walter Cybulski
>National Library of Medicine


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