Gavin Stairs on sun 14 jan 01
At 08:16 PM 1/14/01, Khaimraj wrote:
>Question -
>Now if the stain is a low expansion material , will my
>glaze + stain , be lower in expansion ?
>
>Or will the glaze simply surround the pigment and remain
>at the glaze's original expansion ?
>
>Or have I formed a different situation , and am now
>dealing with another type of structure ?
Hi Khaimraj,
If the stain is encapsulated (i.e., zirconate of some sort) and does not
dissolve, then the resulting glaze will have expansion which is like the
linear sum* of the glaze and stain. However, if the disparity is great,
there may be a problem of micro-fissuring, resulting in a weak glaze,
possibly somewhat opaque, possibly somewhat matte, depending on the
proportions. This is by the physics of mixtures, and not chemical at
all. Well, the degree of fissuring may depend on the adhesive properties
of the two phases.
The second case is when the stain is wholly dissolved in the glaze. In
this case, there is no problem of matting, opacity, etc., as a pure
solution is transparent and homogenous (well, it may be opaque to some
degree), and there is no problem of micro-fissuring either, as there is
only one phase. Also, by the fundamental assumption of glaze calculation,
the expansion will be the linear sum of the constituents.
The final case is when the solution is not complete, or there is a mixture
of phases. Then there may be all sorts of strange interactions, and the
phases may have different properties. However, the mean expansion should
still be the linear sum.
Where the linearity breaks down is where there is an exchange of
constituents (i.e., solution to some degree) and crystallization. Then the
smoothing effect of the glass on the expansion properties is no longer
operational, and some abberent phases may appear, like the quartz phases,
which have different properties than the initial assumption. That's why
the glaze calculations fail in calculating bodies. That and the effect of
pores.
All in all, the situation is no different from that of a highly colored
glaze. Variegated glazes may act somewhat abnormally, but the normal
studio experience is that they can be approximated by normal, homogenous
calculations. As usual, the final word is test, test, test.
Please note that the fundamental assumption of glaze calculation is only an
approximation. A phenomenological accident, as it were. There's no real
reason to expect it to be true, except that the diameters of ions turn out
to be somewhat constant over a wide variety of bonding species. This is
notionally surprising, since our understanding of ions is that they are not
round, and therefore don't really have a constant diameter. The closer
statement would be that the bond length between any two ions is consistent
with there being an ionic diameter, to first approximation, in most
cases. Waffle, waffle waffle. If an ion undergoes an oxidation or
reduction, its diameter will also change. So, reducing a glaze may affect
its expansion, and the glaze density. Similarly, crystals have different
structures from glasses, and may also have different oxidation states, or
bond lengths. Also, they may have more or less strain in their bonds. It
gets complicated.
So, taking the last paragraph into account, we understand that the glaze
calculation is approximate, and within the limits of that approximation,
several of the departures from linearity may hide undetected. Some may be
detected optically, though. So make your glaze, do expansion tests on the
target body, and observe the glaze under magnification and by naked eye, to
see if any of the predicted effects occur.
Gavin
* Linear sum: Given two materials A and B, with expansions eA and eB, and
proportions pA and pB, the resultant expansion will be calculated by
eAB=(eA*pA +eB*pB)/(pA+pB), where pA and pB are mole fractions.
Khaimraj Seepersad on sun 14 jan 01
Good Day to All ,
Hello Glaze Gurus ,
I wish to understand this -
If I have a clear glaze of a given high expansion ,
and I add 10 / 20 / 30 % stain to it as -
Example -
70 % clear glaze to 30 % stain .
Question -
Now if the stain is a low expansion material , will my
glaze + stain , be lower in expansion ?
Or will the glaze simply surround the pigment and remain
at the glaze's original expansion ?
Or have I formed a different situation , and am now
dealing with another type of structure ?
I hope I have explained this clearly enough .
Anyone willing to chance an explanation ?
Khaimraj
Ron Roy on tue 16 jan 01
Hi Khaimraj,
I think Gavins explanation is right on - just to summarize. Yes stains and
colours will affect expansion - either way depending on the expansion of
the materials in the stain or colour. If those materials become part of the
melt they will affect the expansion as any material would. If they remain
mostly aloof (do not melt into the glaze - like oppacifiers) they will
affect the expansion but not as much and it depens on how much is added.
Anything you put into a glaze that is still there after the firing will
affect fit - the one exception would be - if the materials added had
exactly the same expansion on heating/contraction on cooling - as the glaze
there would be no change. What are the chances of that? Maybe the same as
finding two snow flakes the same.
RR.
>I wish to understand this -
>
>If I have a clear glaze of a given high expansion ,
>and I add 10 / 20 / 30 % stain to it as -
>
>Example -
>
>70 % clear glaze to 30 % stain .
>
>Question -
>Now if the stain is a low expansion material , will my
>glaze + stain , be lower in expansion ?
>
>Or will the glaze simply surround the pigment and remain
>at the glaze's original expansion ?
>
>Or have I formed a different situation , and am now
>dealing with another type of structure ?
>
>I hope I have explained this clearly enough .
>Anyone willing to chance an explanation ?
>Khaimraj
>
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Ron Roy
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