Nikom Chimnok on thu 21 aug 97
Greetings,
We are spraying greenware and single firing to cone 1, but unless the
glaze is very thin, it crawls. I would appreciate suggestions on how to
modify the glaze to prevent crawling. The formula is:
soda feldspar 45
Barium Carbonate 10
zinc oxide 5
whiting 15
kaolin 10
silica 15
This is the base--we then add iron or copper for color. This glaze is
used for planters and fountains--it doesn't need to be foodsafe.
I will appreciate any comments.
Nikom
koratpot@korat.loxinfo.co.th
David Hewitt on sat 23 aug 97
Hamer's Dictionary of materials and Techniques' makes interesting
reading in this case. I quote:-
'Glazes which crawl have a high surface tension and are viscous when
molten. A high alumina content makes a glaze viscous. Tin oxide and
other opacifiers tend to increase surface tension and thus encourage
crawling. Zinc oxide is a particular culprit because it causes pre-
firing shrinkage and pulls a glaze together during fusion.'
'To cure crawling, it may be necessary to alter a glaze recipe. A glaze
which contains a high proportion of clay can be altered in a few ways:
some of the clay can be replaced by feldspar; some can be calcined to
render it non-plastic; and some or all can be replaced by a clay of low
shrinkage. Some china clays are available which have a low shrinkage
from wet to dry. This type should be used. With reference to calcining,
zinc oxide gives far less trouble if it is is calcined with some of the
china clay before adding both to the glaze batch.'
According to exactly the analysis of the materials you are using, your
recipe may have an analysis like the following:-
K2O .04
Na2O .16
CaO .47
BaO .15
ZnO .18
Al2O3 .35
SiO2 2.43
This is relatively high in alumina and so you may like to try reducing
the china clay and increase the silica.
Calcining the zinc and the china clay and keeping the recipe the same
may be a better course if it works as the recipe essentially remains the
same.
Your material supplier should be able to advice on a low shrinkage china
clay.
In message , Nikom Chimnok writes
>----------------------------Original message----------------------------
>Greetings,
>
> We are spraying greenware and single firing to cone 1, but unless the
>glaze is very thin, it crawls. I would appreciate suggestions on how to
>modify the glaze to prevent crawling. The formula is:
>
>soda feldspar 45
>Barium Carbonate 10
>zinc oxide 5
>whiting 15
>kaolin 10
>silica 15
>
>This is the base--we then add iron or copper for color. This glaze is
>used for planters and fountains--it doesn't need to be foodsafe.
>
>I will appreciate any comments.
>
>Nikom
>koratpot@korat.loxinfo.co.th
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
URL http://digitalfire.com/education/people/hewitt.htm
LINDA BLOSSOM on sun 24 aug 97
Dear Nikom,
Are you adding a binder to the glaze? Add 1% Veegum to the dry glaze and
then sieve - 80 mesh. If it is already mixed, mix the veegum to water, use
a blender to mix it well and add to the glaze. I have never had a glaze
crawl and I single fire everything. I do not modify the glazes for single
fire and I have used A LOT of glazes.
Linda Blossom
2366 Slaterville Rd.
Ithaca, NY 14850
607-539-7912
blossom@lightlink.com
http://www.artscape.com
Ron Roy on sun 24 aug 97
I agree with all that David Hewitt has pointed out in his reply re this
glaze crawling - there may be another solution however.
Many glazes "crack up" while being fired - I would say most develop e some
breaking of the glaze surface - most glazes "heal" over because that is the
natural tendency - glaze wants to get back together when it gets fluid. The
high surface tension that David talks about has the effect of preventing
that "getting back together."
I would also recommend calcining the Zinc oxide - but the following should
be tried before - just to see if it will work with the materials as they
are.
One way of addressing the problem is to prevent, or at least reduce, the
amount of cracking that occurs during the sintering process. Using clays in
the glaze which form a stronger surface will sometimes be a relatively
painless way of curing the problem and keeping the glaze looking the same.
Ball clays are stronger in this respect than Kaolins. With the addition of
some Bentonite the surface becomes even stronger. I would like to suggest
the following recipe which has the Kaolin replaced by Ball. Ball clay
brings less alumina than Kaolin so ... you need more of it. At the same
time Ball clay brings more silica than Kaolin so the Silica has been
reduced by the proper amount. This has been done on the molecular level so
that the glaze should look the same - well perhaps a little darker because
ball clays have more iron - I don't think this will be a problem - when
choosing between ball clays for this substitution choose those with the
least iron in their analysis. Keep in mind that clays with a high MOR
(modulus of rupture) will work best in this situation. Discriminating on
the basis of how hard it is to break any given ball clay (or Kaolin for
that matter) would be the best strategy in this case. What I have said
means of course I think any ball clay would be better than any Kaolin - I
expect and welcome any comments on this brazen assertation
Original cone 1 recipe which crawls when spayed on more than thin.
>soda feldspar 45
>Barium Carbonate 10
>zinc oxide 5
>whiting 15
>kaolin 10
>silica 15
Total 100.0
Ratio 6.71
My revision using glaze calculation software.
soda feldspar 45
Barium Carbonate 10
zinc oxide 5
whiting 15
Ball clay 13
silica 12
Bentonite 1
Total 101.0
Ratio 6.75
While this suggestion seems directly opposite to that below I have actually
cured crawling glazes using this method - so I know there is a possibility
of success.
>This is relatively high in alumina and so you may like to try reducing
>the china clay and increase the silica.
>Calcining the zinc and the china clay and keeping the recipe the same
>may be a better course if it works as the recipe essentially remains the
>same.
>Your material supplier should be able to advice on a low shrinkage china
>clay.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
Tracy Bradford on wed 19 sep 07
Hmmm, I've been using a glaze for about twelve years now with lots of
success. I've recently switched clay bodies this past spring and now
I'm having problems with crawling with this particular glaze only. I
use about seven different glazes, with six being my own recipe and no
problem with crawling. The "problem" glaze is a commercial mix. I've
tried the normal procedures for curing the problem with no success.
Here's what I've tried: soaking then sponging the pieces in water.
Blowing air off with a compressor and vacuuming the pieces. Still no
luck and I should point out that all the items that crawl are of
course the larger or more time consuming intricate pieces. Sigh. I
bisque fire the pottery and glaze fire to cone six. Any suggestions?
Tracy
Edouard Bastarache Inc. on wed 19 sep 07
Acording to Robin Hopper :
Crawling is caused by a high index of surface
tension in the melting glaze. It is triggered by
adhesion problems, often caused by bad
application. It occurs where a glaze is
excessively powdery, and does not fully adhere to
the surface of the clay. This can be alleviated by
the addition of a small amount of gum to the glaze
batch. Crawling is more common in matt glazes than
in fluid ones; sometimes the problem of crawling
can be adjusted by the addition of a small amount
of extra flux. Crawling can also occur when one
glaze is applied over another, particularly if the
first is allowed to dry out completely before the
second application. Some fluxes, particularly zinc
and magnesium, are likely to cause crawling when
used in excess. Calcining part of the zinc can
help this problem (Robin Hopper in The Ceramic
Spectrum)
La rétraction est provoquée par un index élevé de
tension superficielle de la glaçure en train de
fondre. Elle est déclenchée par des problèmes
d'adhésion, souvent provoqués par une mauvaise
application. Il se produit quand une glaçure est
excessivement pulvérulente (poudreuse), et
n'adhère pas entièrement à la surface de l'argile.
Ceci peut être diminué par l'addition d'un peu de
gomme au bain de glaçure. La rétraction survient
plus fréquemment dans les glaçures mates que
fluides; parfois le problème de la rétraction
peut être ajusté par l'addition d'un peu de
fondant supplémentaire. La rétraction peut
également se produire quand une glaçure est
apliquée par-dessus une autre, en particulier si
la première était complètement sèche avant la
deuxième application. Quelques fondants, en
particulier le zinc et le magnésium, sont
susceptibles de causer de la rétraction lorsqu'utilisés
en excès. La calcination d'une partie du zinc ou l'utilisation
de zinc calciné comme tel peuvent aider ce
problème (Robin Hopper in The Ceramic Spectrum)
Gis la revido,
(A la revoyure)
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
http://perso.orange.fr/smart2000/livres.htm
http://www.pshcanada.com/Toxicology.htm
http://www.ceramique.com/librairie/
http://www.flickr.com/photos/30058682@N00/
http://myblogsmesblogs.blogspot.com/
Maggie Jones on wed 19 sep 07
vee-gum and the like.
maggie
On Wed, 19 Sep 2007 13:07:16 -0400 Tracy Bradford
writes:
> Hmmm, I've been using a glaze for about twelve years now with lots
> of
> success. I've recently switched clay bodies this past spring and
> now
> I'm having problems with crawling with this particular glaze only.
> I
> use about seven different glazes, with six being my own recipe and
> no
> problem with crawling. The "problem" glaze is a commercial mix.
> I've
> tried the normal procedures for curing the problem with no success.
>
> Here's what I've tried: soaking then sponging the pieces in water.
> Blowing air off with a compressor and vacuuming the pieces. Still
> no
> luck and I should point out that all the items that crawl are of
> course the larger or more time consuming intricate pieces. Sigh.
> I
> bisque fire the pottery and glaze fire to cone six. Any
> suggestions?
>
> Tracy
>
>
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Tracy Bradford on wed 19 sep 07
Thanks Edouard and Maggie,
Vee gum... of course! I should have thought of that sooner.
Tracy
Ron Roy on thu 20 sep 07
Actually crawling is a two stage problem and the answers to the problem can
be on two different levels or both levels at the same time.
To understand crawling you need to get into the kiln with your pots and
keep an eye on them during the firing. Impossible I know but if your
understanding is complete enough you can do it.
Most glazes crack during sintering in the kiln - sometimes before they even
get into the kiln - having a magnifying glass and using it to look at your
glazes before firing will help you know which ones may be problematic.
So the cracks usually happen - and sometime the glaze peels away at the
edges of the cracks - sometimes even falls off.
You can deal with it at the wet glaze stage by using different materials -
ball clay subbed for kaolin for instance - because ball clay forms a much
tougher skin than kaolin does.
Sometimes the answer is as simple as waiting till the glaze is completely
dry before firing - so that boiling water does not "lift" the glaze up.
The second part is - when the glaze is finished melting - usually it wants
to get back together with itself and cover over the bare clay. Some (most)
glazes will flow easily and do get together - others are stiff and just
don't have enough time to get together. If the crawling is minimal perhaps
a short soak or slow cool is all that is needed.
Our oxides can be grouped by viscosity and surface tension - if a glaze is
too stiff for instance decreasing alumina will help it flow better - so one
answer to crawling is to lower the clay in a recipe.
There are two lists in the Hamer book - viscosity and surface tension -
read those and the section on crawling and you will have the tools you need
to deal with crawling.
RR
>Acording to Robin Hopper :
>
>Crawling is caused by a high index of surface
>tension in the melting glaze. It is triggered by
>adhesion problems, often caused by bad
>application. It occurs where a glaze is
>excessively powdery, and does not fully adhere to
>the surface of the clay. This can be alleviated by
>the addition of a small amount of gum to the glaze
>batch. Crawling is more common in matt glazes than
>in fluid ones; sometimes the problem of crawling
>can be adjusted by the addition of a small amount
>of extra flux. Crawling can also occur when one
>glaze is applied over another, particularly if the
>first is allowed to dry out completely before the
>second application. Some fluxes, particularly zinc
>and magnesium, are likely to cause crawling when
>used in excess. Calcining part of the zinc can
>help this problem (Robin Hopper in The Ceramic
>Spectrum)
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ron Roy on thu 20 sep 07
Hi Tracy,
Make sure the glaze is completely dry before firing - that may help.
What does the maker of the glaze say? Binders can work - bentonite will
help. I'm not recommending soluble materials because they some times bring
their own problems.
RR
>Hmmm, I've been using a glaze for about twelve years now with lots of
>success. I've recently switched clay bodies this past spring and now
>I'm having problems with crawling with this particular glaze only. I
>use about seven different glazes, with six being my own recipe and no
>problem with crawling. The "problem" glaze is a commercial mix. I've
>tried the normal procedures for curing the problem with no success.
>
>Here's what I've tried: soaking then sponging the pieces in water.
>Blowing air off with a compressor and vacuuming the pieces. Still no
>luck and I should point out that all the items that crawl are of
>course the larger or more time consuming intricate pieces. Sigh. I
>bisque fire the pottery and glaze fire to cone six. Any suggestions?
>
>Tracy
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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