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Non-toxic investment powder


was: Future of hand craftsmanship?

Olivine sand is one of the suggested substitutes, fine alumina
sand will also work. I think you will find that there are not many
cheap substitutes. 

I was thinking about Olivine; I wonder if it’s available as a fine
powder. I’ve purchased olivine sand for abrasive blasting puposes,
aluminum oxide as well.

The problem is that any sand that is fine enough to work will pose
the same dangers to the lungs. It is not that silica is inherently
poisionous it is that it is not soluble by the body's fluids so it
stays in the lungs if the particle size is small enough and also it
is so sharp that it scars the lung tissues. I believe one of the
"advantages" of olivine is it is a smoother particle but that does
not address the size issues. 

[With all due respect, I’m not sure that’s entirely true. Some
dusts, like silica, are considerably worse than others, regardless
of particle size, although it is the smallest particles that do the
most damage.

From what I’ve read, there is a chemical reaction that occurs in the
lungs when freshly-fractured silica contacts the moist membranes of
the lung. This results in the formation of silicic acid, which is
corrosive, and can lead to sudden death (from acute silicosis); this
has been documented in the cases of some unfortunate sand-blasters
who weren’t sufficiently protected. The Gauley Bridge disaster of
1936 was an example of what can happen when unprotected workers are
exposed to large amounts of crystaline silica - of 2000 workers
tunneling through a quartz mountain, about 400 died on the spot; 1500
more suffered from severe silicosis thereafter. Apparently, the
chemical reactions with freshly-cleaved silica particles in the lung
can even cause cancer. This is why olivine, aluminum oxide, etc. are
now sold instead of silica sand for abrasive blasting, even though
they are considerably more expensive, and why bags of silica sand
come with warnings against using it for blasting. OSHA recommends
using a substitute material, and provides a handy chart:

Here’s one medical account of silica’s cytotoxicity mechanism:

Pathophysiology: Small (A31 mm) particles are more dangerous
because they are more likely to be deposited distally in the
respiratory bronchioles, alveolar ducts, and alveoli. The
surface of these particles generates silicon-based radicals that
lead to the production of hydroxyl, hydrogen peroxide, and other
oxygen radicals that damage cell membranes by lipid peroxidation
and inactivate essential cell proteins.

Alveolar macrophages ingest the particles, become activated, and
release cytokines, including tumor necrosis factor,
interleukin-1, and leukotriene B-4, as well as chemotactic
factors that recruit other inflammatory cells. The ensuing
inflammation damages resident cells and the extracellular
matrix. Transforming growth factor-alpha induces proliferation
of type 2 pneumocytes, and other cytokines (eg, platelet-derived
growth factor, insulinlike growth factor) stimulate fibroblasts
to proliferate and produce collagen; fibrosis results. Silica
particles outlive the alveolar macrophages that ingested them,
thereby continuing the cycle of injury.

This damage to the macrophages (which normally remove particles from
the lungs) can apparently result in autoimmune diseases like
arthritis and lupus, and contribute to renal failure and cardiac

I’m not sure the dust inhaled when breaking out investment molds is
quite as bad, but silica (along with talc, mica and asbestos) is
considered a “fibrogenic” dust, which means it promotes the growth
of fibrous tissue in the lungs, which doesn’t work the same as
normal lung tissue. The silica powder used in investment mixtures is
produced by milling crystalline quartz; although aging might
ameliorate some of the worst effects, investment is usually
recommended to be used when fresh. I’m not sure if the burn-out
process counts as aging, or if it recharges the ionic activity of
the silica particles.

Compared with silicosis, which is a well-known hazard, the effects
of olivine inhalation seem less certain. While I cartainly wouldn’t
recommend inhaling any type of dust if you can help it, some
investigators find it to be more benign, at least in rodents (which
seem to be more sensitive to it than humans). See and

Another study comes to a different conclusion:

However, “no disease or pneumoconiosis from olivine minerals has been
observed in humans”, according to Applications and Computational
Elements of Industrial Hygiene, by Martin B. Stern

Aluminum oxide (corundum, or Al2O3) is another promising candidate.
At least according to one MSDS, it’s considered at worst a “nuisance
dust” with no particular toxicity:

It’s more chemically inert than olivine, and isn’t a silicaceous
mineral at all. It’s more expensive than olivine, but not totally
prohibitive in cost. In general, it seems like a good candidate for
replacing silica flour, since “Aluminum Oxide dusts have caused no
known systemic or pathological problems. The material is inert in
the body. Some individuals may experience allergic sensitivity
reactions. These are generally limited to mild occupational
dermatitis. Chronic inhalation may result in pleural plaques not
associated with cancers. Other effects principally derived from
physical abrasion.” - (Zircar MSDS) So, having looked into it, it’s
looking like aluminum oxide is the most promising replacement for
silica flour in investment powder, if I can locate a reasonable
source of supply. I’ll do some experiments and see how it works, and
get back to you all…

Andrew Werby

Here are some more resources on silicosis, if you’re interested: (a rundown of the silicosis issue in
(If it’s silica, it’s not just dust)
(this gives an overview of various types of dusts, and their health

#2 Here is
the direct link for the silica free investment. The amounts are too
small to be useful for a serious caster though. 

I think you are confusing two different products, here is their
description of the above product from their website.

"Our Ultra Smooth Investment is a premium jewelers' investment
designed for use with platinum, gold and silver and is stable at
temperatures in excess of 2000F. Investment is a product that is
specially blended from gypsum and crystobilite for use in casting
metals. Our Ultra Smooth Investment produces pellets and
placeholders that have no porosity (pin holes) so your rings
shanks are smooth inside after firing, requiring less cleanup and
polishing. Pellets made from Ultra Smooth Investment do not shrink
and do not stain the metal. 

and from further down the page

Our Silica-Free Casting Powder is an alternative to our
traditional investment. Silica-Free Casting Powder does not contain
crystobolite. It can be mixed without the use of a dust mask.
Pellets and placeholders made from Silica-Free Casting Powder are
not as strong as investment, require 2 hours to set up, but are

The "Silica-Free Casting Powder appears to be just a non silica
based castable ceramic material not a jewelry casting investment.


James Binnion
James Binnion Metal Arts




Thanks that I a lot more than I had on the actions of
silica in the lungs. It will take a while to read it all.


James Binnion
James Binnion Metal Arts



Hi Andrew,

I use alumina (calcined AKA reactive alumina as it aids in binding)
in the investment powder I use for glass casting, hot strength is as
good as investment made with silica flour, the alumina I use is of a
similar mesh to silica flour too btw.

Surface reproduction is very good too.

Just my first hand observations…

Oh yes, it is about 50 to 60% more expensive, but the increase in
safety in my workspace is immeasurable. (I pay AU$80/20kg bag which
will last me about 6 months or so.) That said; I do wear a respirator
when I mix the stuff as I just don’t like the idea of breathing any
kind of dust.