daniel on wed 31 mar 04
Hi All,
Thinking on the recent punch a hole in it/whack a glass liner in it/use a
ziplock bag in it threads I am wondering if a low fire vitreous liner cannot
be made. While I realise that non-vitrified clay will absorb water leading
to what I think of as absorption-based crazing (rather than COE mismatch
crazing and feel free to beat me up on that. I haven't got the theory on it
down yet) I wonder the following. It seems that a glaze crazes because it
bonds to the body and the body changes size after the glaze has rigidified
to some extent. It seems that if one could formulate a glaze that could be
applied to a pot, which would fire to a vitrified glass but which would not
bond to the clay, that it would be better able to resist crazing effects of
body size changes. I grant that this will not confer on the piece the other
advantage of a decent glaze, viz greater strength. Just a thought.
And no, I have no idea how to make such a glaze.
Thanx
D
Jon Pacini on thu 1 apr 04
Greetings All ---- Hi Daniel,
You wrote--- It seems that a glaze crazes because it
bonds to the body and the body changes size after the glaze has rigidified
to some extent. It seems that if one could formulate a glaze that could be
applied to a pot, which would fire to a vitrified glass but which would not
bond to the clay, that it would be better able to resist crazing effects of
body size changes.
To a certain extent you are correct in thinking that porous bodies can
absorb water and cause a glaze to craze. It doesn t always happen, but it
can. Physical swelling would seem to be mechanically impossible considering
that fired clay is a relatively rigid material. I ve never been able to
physically measure this swelling phenomenon, but some sort of pressure
is exerted on the glaze when water is present in fired clay.
I don t feel the problem of porous/crazing is solely an adhesion problem,
but rather a combination of lack of adhesion and more importantly, an
absence of elasticity within the glaze.
Let s look at adhesion first. In low fire situations using typical
earthenware, there is not a great deal of interaction/bond between the clay
and glaze. The glaze almost floats on the surface of the clay like paint,
rather than having the interface layer that higher fired ceramics exhibit.
Because of this lack of adhesion, you can very easily go from a crazing
glaze to one that shivers or even shells off the ware wholesale. So
attempting to lessen that bond, as you ve suggested, would in fact create a
greater problem. What is needed is a way to increase the interface layer
between the clay and glaze. You can do this by increasing the fusion of the
body either through formulation or firing hotter. The more glasslike a body
can become, the more interaction there is with the glaze. However by doing
this you would usually see a decrease in the absorption. It becomes a dance,
how much absorption do you want, how much interaction.
So that leaves elasticity. Pressure is what is causing the crazing, or
rather lack of, so the glaze needs to stretch. Since we re not really
dealing with a COE issue here, how elastic a glaze is, becomes the issue.
Elasticity is a tough nut to crack. Lead used to be the cure all in low
fire glazes. Not that there aren t examples of crazed lead glazes, but lead
made glaze fit much easier because of it s apparent elasticity. Boron in
small percentages seems to impart some elasticity, there may be other
examples. I m not aware of how much work has been done on this topic. But I
d bet that a glaze can be designed to accomplish sufficient elasticity, and
more than likely, is already out there being used.
Best regards
Jon Pacini
Clay manager
Laguna Clay Co
daniel on thu 1 apr 04
Hi Jon,
Warning long post.
Thanx for the thoughts on this. My responses interleaved.
> Physical swelling would seem to be mechanically impossible considering
> that fired clay is a relatively rigid material. I ve never been able to
> physically measure this swelling phenomenon, but some sort of pressure
> is exerted on the glaze when water is present in fired clay.
This has always been my problem with the water absorption causes crazing
argument. I could not see how a now-rigid (at least mostly so) body would
alter its size and shape. I do not understand this. In fact I can't yet feel
100% comfortable that delayed crazing is due to absorption as I've yet to
see a good explanation of HOW it happens. I don't dispute that crazing
occurs after time but I don't understand it.
>
> I don t feel the problem of porous/crazing is solely an adhesion problem,
> but rather a combination of lack of adhesion and more importantly, an
> absence of elasticity within the glaze.
Elasticity occurred to me and the thought occurred of lining a piece with a
"high fired rubber" whatever that is. I also thought that with no adhesion
the glaze would like slump into the bottom of the piece during firing.
Perhaps roughing up the surface would help it hold on. Elasticity seems only
to be a problem where the change of size of the body impinges on the glaze
in some way. This could be because of the shape eg.a ledge or ridge or
because of the bonding between the body and the glaze. Thus went my thinking
anyhow. My simplified model was a cylinder with virtually no internal
features. I was dodging the outside for now and the real shape of vessels.
So I was imagining a glass cup being fired inside a clay one.
>
> Let s look at adhesion first. In low fire situations using typical
> earthenware, there is not a great deal of interaction/bond between the clay
> and glaze. The glaze almost floats on the surface of the clay like paint,
> rather than having the interface layer that higher fired ceramics exhibit.
> Because of this lack of adhesion, you can very easily go from a crazing
> glaze to one that shivers or even shells off the ware wholesale. So
> attempting to lessen that bond, as you ve suggested, would in fact create a
> greater problem.
This section has caused a swirling of thoughts. I'll try to get them
straight. Leaving aside COE and water for a minute, it seems that crazing is
dependent on a greater adhesion of the glaze to the pot and lesser adhesion
of the glaze to itself, parallel to the pot surface. Elasticity being
considered neglible (ie. there isn't any) at least for now. It is also
assumed that the clay body of the piece is generally stronger.
It seems to be a given (and on this challenge as you see fit) that a higher
fired glaze is "stronger" than a lower fired one. What does this mean ? For
our purposes here does it mean that the glaze to glaze bonding is greater
than for a low fire glaze ? If so then one would imagine that the glaze
would inherently resist crazing more than a lower fired "weaker" glaze.
Having said that though, the glaze to pot bond is more highly developed
also. Thus we basically have a greater irresistable force meeting a larger
immovable object, so to speak, in the high fire case. This would suggest
that in order to produce a high fire decorative crackle (a little glaze
project I want to try for myself soon) that the difference between COEs of
the body and glaze would have to be proportionally greater than in the low
first case. Does this make sense ?
I fear I have strayed from the point somewhat.
> What is needed is a way to increase the interface layer
> between the clay and glaze. You can do this by increasing the fusion of the
> body either through formulation or firing hotter.
My original gambit on this was that a low fire liner that did not craze was
simply chemistry and that one could do it with something like a majolica
glaze. This led me to the problem of delayed crazing as utlimately causing
the piece to fail to hold water. (Ah there it is, I found the point again:))
Perhaps nothing last forever and we should live with it.
> The more glasslike a body
> can become, the more interaction there is with the glaze.
Ultimately though it seems that the more glasslike a body becomes the higher
the firing is. This seems to lead us to high fire if we want to avoid
crazing.
> So that leaves elasticity. Pressure is what is causing the crazing, or
> rather lack of, so the glaze needs to stretch. Since we re not really
> dealing with a COE issue here
So as we are both avoiding COE here as much as possible what is the source
of the pressure that causes the crazing ? Absorption - see above for my
confusion on that, or is there another source ?
Oh hang on a minute ! Are you suggesting (or if not, could it be possible)
that the lack of elasticity in the glaze and just relatively slight changes
in temperature - those of normal use - could slowly open small crazes that
grow over time ? Particularly if the body and the glaze receive different
heating. In this case I can then see how elasticity would really help.
Thanx
D
Ron Roy on fri 2 apr 04
Hi Daniel - what would be interesting is - what would support the glass if
it were not attached to the clay?
RR
> Thinking on the recent punch a hole in it/whack a glass liner in it/use a
>ziplock bag in it threads I am wondering if a low fire vitreous liner cannot
>be made. While I realise that non-vitrified clay will absorb water leading
>to what I think of as absorption-based crazing (rather than COE mismatch
>crazing and feel free to beat me up on that. I haven't got the theory on it
>down yet) I wonder the following. It seems that a glaze crazes because it
>bonds to the body and the body changes size after the glaze has rigidified
>to some extent. It seems that if one could formulate a glaze that could be
>applied to a pot, which would fire to a vitrified glass but which would not
>bond to the clay, that it would be better able to resist crazing effects of
>body size changes. I grant that this will not confer on the piece the other
>advantage of a decent glaze, viz greater strength. Just a thought.
>
>
> And no, I have no idea how to make such a glaze.
>
>Thanx
>D
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Jon Pacini on fri 2 apr 04
Greetings all ---Hi Daniel----- you wrote and I ve tried to answer, again
interspersed
I could not see how a now-rigid (at least mostly so) body would
alter its size and shape. So as we are both avoiding COE here as much as
possible what is the source
of the pressure that causes the crazing ? Absorption - see above for my
confusion on that, or is there another source ?
There are models for rehydration of clay after it has been fired. Like most
models there are exceptions and there are degrees to which it is applicable.
COE driven expansion is thermally driven and I don t believe it is
applicable in a case where the temperature does not play a part in
expansion. If this type of expansion were temperature driven, then both the
clay and glaze would expand/contract at equivalent rates, unless you had
inappropriate COEs. But in this rehydration model, only the clay is
expanding and the glaze remains the same . My apologies to Robert Plant.
I also thought that with no adhesion
the glaze would like slump into the bottom of the piece during firing.
Well, that is a possibility if the glaze had no surface tension and was very
fluid. But my concern would be that the glaze would be so stiff that it
would fuse to itself but not at all to the ware. It would shiver or shell
off whole. I ve seen engobes do this and under fired glazes do it too. It
becomes a vessel unto itself. Come to think of it, this might be a way to
produce that separate liner that you re looking for. In Glass there is a
technique called Pate de Ver where powdered glass is packed in a clay mold
and removed after fusing.
Actually I think a vitreous engobe might be the resolution to the entire
dilemma. It could be an intermediate layer, which would fit both
materials, the clay and the glaze and be impervious to water.
It seems to be a given (and on this challenge as you see fit) that a higher
fired glaze is "stronger" than a lower fired one. What does this mean ? For
our purposes here does it mean that the glaze to glaze bonding is greater
than for a low fire glaze ? If so then one would imagine that the glaze
would inherently resist crazing more than a lower fired "weaker" glaze.
In a general way high fire glazes do become harder than low fire ones. It
has to do with the silica/alumina/alkaline ratios. Alkaline fluxes are soft
and alumina is hard, because you are letting heat do a lot of the work of
the flux you can add more alumina to high fire glazes. Once again there are
always exceptions, but I don t think this is a weaker/ stronger issue. You
can have a strong glaze, but it has no give, it s brittle. This is a brittle
versus elastic issue. What you want is a glaze that has the ability to
give ---elasticity. It s not necessarily weaker.
Ultimately though it seems that the more glasslike a body becomes the higher
the firing is. This seems to lead us to high fire if we want to avoid
crazing.
Firing hotter is certainly an easy solution. But it may not be better or may
not always be an option. If for whatever reason you are locked into porous
clays or you just like the challenge of using them, then being given the
advice that you should fire hotter doesn t really resolve your dilemma.
Fluxing the body more is a possible option, but I think we still end up
going back to the elasticity issue as long as the body is absorbent.
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co
daniel on fri 2 apr 04
Hi Jon, All,
> There are models for rehydration of clay after it has been fired. Like most
> models there are exceptions and there are degrees to which it is applicable.
If you have any sources I could look up I'd be interested.
> But my concern would be that the glaze would be so stiff that it
> would fuse to itself but not at all to the ware. It would shiver or shell
> off whole. I ve seen engobes do this and under fired glazes do it too. It
> becomes a vessel unto itself. Come to think of it, this might be a way to
> produce that separate liner that you re looking for.
This is exactly where my thought started. Fire a liner glass that is not
bonded to the body but has the same shape and is formed inside the clay body
of the piece which is also matured in the same firing ie. make you water
proof insert when you make the piece, but as an insert.
> In Glass there is a
> technique called Pate de Ver where powdered glass is packed in a clay mold
> and removed after fusing.
I'll look it up.
>
> Actually I think a vitreous engobe might be the resolution to the entire
> dilemma. It could be an intermediate layer, which would fit both
> materials, the clay and the glaze and be impervious to water.
>
They use something similar do they not on electrical porcelain - a terra sig
to match glaze to clay body ? I wonder ... hmmm test thoughts occur.
Thanx
D
daniel on fri 2 apr 04
Hi Ron,
> Hi Daniel - what would be interesting is - what would support the glass if
> it were not attached to the clay?
>
On the adhesion/slumping/supporting form thing if the liner does not bond
to the body I confess to having no solution. Would a stiff glass melt be
possible at cone 06 ? even cone 6 or 10 ? I have no solution to this unless
some sort of mechanical support could be built into the form like ledges.
This would then rely on the surface tension of the melt to hold the glass
together. Lord knows what I do for the inside of a collared form or bottle
:)
Thanx
D
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