Molecular level difference between vitreous and devitrified silica from Digitalfire.com |
You can see that there is not much difference between the the two states of the glass in structure, but mainly the arrangement of molecules.
The appearance of devitrification has a range of appearances
from a mild smeary look through a dull surface to a crazed, crumbly aspect in
severe cases.
Mild devitrification |
Medium level devitrification requiring abrasive cleaning |
Causes of devitrification are related to slow changes of
temperature (up or down) and most importantly nucleation points such as dust,
oils, or cleaning residues. So, thorough cleaning is most important.
Causes in repeated firings of the same piece relate to:
Cleaning.
It is important to thoroughly clean the piece before each
subsequent firing. Many times abrasive
cleaning such as sandblasting is important to clean out impurities from the
previous firing. The resulting surface
from any abrasive cleaning requires further cleaning with lots of clean water
and a thorough drying with clean cloths or paper.
Slow cooling or heating.
Devitrification normally occurs in the range of 670⁰C to
750⁰C. This is the reason for the rapid rates of advance in this temperature
range rather than other factors. It can
form both on the rise and on the fall in temperature. Slower rates in the
devitrification range allow enough time for the crystallisation to begin.
High temperatures.
Both high temperatures and long soaks can promote
devitrification. It is not just the slow
rise or fall in temperature, but long periods at high temperature can lead to
devitrification even though other precautions have been taken.
Changes in the composition
High temperatures and many repeated firings of the piece can
lead to changes in the glass. Some
metals and fluxes are more likely than others to change composition or oxidise
at extended soaks at high temperatures.
This can reduce the ability of the glass to resist devitrification.
Prevention/Correction
Prevention relates to thorough a) cleaning and b) firing rates.
All correction of devitrification relates to the
modification of the surface. If the
problem is only at the surface, you can use either abrasive cleaning or the addition of fluxes to the surface, or a combination of the two.
Where you have a mild dulling of the surface due to
devitrification you can apply a flux.
This softens the surface by reducing the melting temperature of the
glass and so reverses the crystallisation at the surface. The devitrification
solution can be a proprietary spray such as Super Spray. Be aware that some
sprays use lead particles as the flux, so are inappropriate for pieces intended
to be food bearing. You can make your own devitrification solution by
dissolving borax in distilled water. When the devitrification is wide spread or deep, abrasive
cleaning is required.
Abrasive cleaning can be by hand with sandpapers or diamond
pads. Be sure to keep them damp. This keeps dust from rising, and the sanding
surfaces clean for better working.
Sandblasting can be quicker, especially on uneven surfaces or where
there are deep imperfections. The
surfaces resulting from abrasive cleaning need to be scrubbed clean with
sufficient water, and then polished dry as for a finished piece.
It is possible to combine both these methods to be more
certain of a shiny finish. When
combining, you need to do the abrasive cleaning first, then the wet cleaning
and finally add the devitrification solution.
A fourth possibility is to sprinkle a fine but consistently
thick layer of clear fine frit or powder over the piece. This, when fused, provides the new surface concealing
the devitrification below. Again, this
must be done at a full fuse, so it is not applicable to items you wish to remain
tack fused.
However, if the devitrification has progressed to a crazed
appearance, it is so deep as to be almost impossible to reverse. The piece will also probably have developed
incompatibilities. So the only real option in crazed pieces is to dispose of
them. They will not be useable in
combination with any other glass. They will make any glass with which they are
combined subject to devitrification and possible breakage. These are pieces which truly cannot be cut up
and re-used.
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