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studio windows

updated thu 31 oct 96


Gerry Barbe on thu 10 oct 96

My friend had her electric kiln in her basement; it was under a window
which was the only ventilation. After ten years the glass in the window
is so etched it is opaque. I don't know if it's aluminum, but if those
fumes can etch glass, what could they do to lungs?

Regards, Marilyn, in Ailsa Craig, ON Canada


Kerr - M. Christine on thu 10 oct 96

you didnt mention what kind of glaze firing she/he was doing. if the
glaze contained flourine (cryolite immediately comes to mind - also I
think wollastonite also contains flourine but havent bothered to look
that up.) then its very possible that this is what caused the etching of
the glass-HF is used to etch glass commercially. Other than that, i dont
know - is it possible that its the result of the repeated heat too near
the glass? aluminum wont cause etchiing. Or did you mean tha the
aluminum frame on the window also got etched? That could be the result
of any number of fumes - SO2/3 being a couple. HCl another.
Chris Kerr Chemistry, Montgomery college MD on fri 11 oct 96

Today's digest hasn't arrived yet ... my apologies if you already have
the information.

On 8th October, Torgeir Henriksen wrote:

>I have had my studio in the same building for 17 years.I have 3
>electric kilns and a normal ventilation for each of the kilns. All my
>windows look grey just after washing. Is there anything I can do to
>this? Is it the aluminium in the clay which reacts with the glass?
>It will be very expensive to reinstall new windows, but there might be
>something to add to the water when washing?

The culprit is fluorine, as Chris Kerr pointed out. It is present in
significant quantities in cryolite, lepidolite and fluorspar. Most
people don't consciously use any of these minerals, but up to 2%
fluorspar is present in many clays and in the several forms of cornish
stone (aka china stone, cornwall stone, etc). Lepidolite is sometimes
used for its lithium content, but it contains around 5% fluorine, which
can cause pitting and blistering in a glaze due to the release of
gaseous fluorine compounds.

Fluorine is far too reactive to exist uncombined in these circumstances,
and it's pretty certain to be hydrogen fluoride that etches the glass.
(It's not very kind to hot kiln bricks either.) Water from the warm
moist atmosphere of the kiln room condenses on the cold glass of the
window, and the hydrogen fluoride dissolves in it, forming a very dilute
hydrofluoric acid. Hydrofluoric acid etches glass, but as it's so dilute,
you only begin to notice it after several years.

So ..... you can't wash the grey off. It's permanent, and will get
worse. You could coat your window with paraffin wax or a polythene sheet
(but you wouldn't be able to see through it), or perhaps a sheet of
adhesive transparent plastic like the stuff they use to keep the sun
out of glass walled offices. The adhesive might even fill out the etched
surface and render it clear again (if you do it, let us know). Most
people just seem to live with it! In less harsh climates than Norway,
double glazing should prevent the condensation and so protect the
window. (Anybody experienced this? Does it happen to windows in hot, dry

Following up on Marilyn's comment, fluorine and hydrogen fluoride are
VERY poisonous. I don't know what the limits of tolerance are, nor how
it accumulates in the atmosphere of a kiln room. For that reason, and
because I don't like breathing in metal oxides, carbon monoxide or any
of the other nasties that leak out of electric kilns, I switch on an
extractor fan and never work in the kiln room until late in the cooling
cycle. Perhaps our Industrial Health experts could comment?

Kevin - in Leamington Spa, England. (11 miles from Stratford-on-Avon.)

* Kevin Hulmes CROVM4(HULMESK)

* Phone/Fax/Msgs (0)1926 887003 (UK)