Marni Turkel on fri 22 sep 00
I am trying to solve a problem I am having with crawling when using two
glazes, one over another. The top glaze is adhering better to the
underglaze than the underglaze is adhering to the pot. It isn't the top
glaze crawling away from the base, but the two come up together and leave
bare patches of clay. The overglaze is applied quite thinly, but there is a
delicate line between thin enough that it stays put and too thin to give
the desired effect. I'm missing the mark way too often to put this into
production. If it is going to crawl there is sometimes an indication of
small blisters in the raw glaze coat. Occassionally, the blisters are large
enough to leave no doubt and I just wash the glaze off. Applying the
overglaze immediately after the 1st coat is no more successful than waiting
days to apply, so I decided to look at shrinkage. Drying thoroughly before
firing doesn't insure success. I have the same crawling on jiggered c6
B-mix and the c6 casting slip I make. The base glaze contains 13% EPK and
the overglaze contains 15%. When I reverse the glazes there is much less
tendency to crawl, but it can still happen.
I have done this successfully and consistently on production ware some
years ago. Then one day about 6 years ago they just started crawling. I was
unable to solve the problem, so I finally just stopped using them and got
out of the dinnerware business. Both of these glazes used to contain
gerstley borate, but have been recalculate. (I hired Ron Roy to do it, and
he did a great job!) They look even better than the old versions, and they
are generally better to work with. I am determined to make this work.
In the last few days I have tried substituting calcined kaolin for 1/2 of
the EPK in the recipe for the over-glaze while leaving the underglaze
unchanged. The first tests have been good. Now I am wondering if I should
also be looking at different kaolins instead of EPK. Does anyone have any
advice on differing shrinkage or tendency to contribute to crawling?
Matching the % in the two glazes?
I read with interest the posting about Elmer's glue in a sprayed glaze. I
will certainly try it to see if I makes the under glaze adhere better. Are
there any suggestions for additions to the base? I have tried Karo syrup
and CMC gum in years past but never found it to help.
Thank you for any suggestions, and for being a very interesting group of
people, an invaluable source of information and the occassional really good
laugh.
Marni
Marni Turkel
Stony Point Ceramic Design
2080 Llano Rd 1B
Santa Rosa, CA 95407
Pottery: 707-579-5567
Office: 707-579-9511
Fax: 707-579-1116
Jonathan Kaplan on fri 22 sep 00
Marni:
We ran into some similar problems with a glaze that we had used for years.
In general, glazes that are high in clay content do indeed exhibit a
tendency to crawl. This can be further exacerbated by using bentonite in
the glaze. Once we eliminated the bentonite, the glaze began to work well
again.
I was also concerned that there was something not correct with the last
shipment of bentonite we received. I have since eliminated that particular
shipment from any of our glazes.
The idea of using calcined kaolin in both your glazes is on the right track
of eliminating or changing those materials what exhibit a potential to
shrink in the application state as well as crawl in the fired state.
You might try using Pioneer Kaolin instead of EPK if you wish to continue
to use a primary kaolin product.
Or rather, use a ball clay that is specifically blended as a glaze clay.
These are specifically formulated and blended to provide excellent
suspension characterisitics as well as contribute alumina and silica to the
mix, obviously. Specifically, Old Hickory Clay Company makes a few ball
clays that will probably help rectify the problem. There is some wisdom to
eliminate the EPK altogether, as EPK is not really a good glaze clay and
substitute one of these very clean burning specifically formulated ball
clays instead. I think you will appreciate the results.
As a side bar, Old Hickory also produces a series of specifically
formulated ball clays for casting, blended with the right particle sized
materials to increase the casting rate. Their FC 340 is now a standard
ingredient in all our casting slips. FC 340 is available from Mile Hi in
Denver under the private label brand of MH77.
Best
Jonathan
Jonathan Kaplan, president
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477
voice and fax 970 879-9139
jonathan@csn,net
http://www.sni.net/ceramicdesigin
Plant Location:
1280 13th Street Unit 13
Steamboat Springs CO 80487
(please use this address for all deliveries via UPS, comman carrier, FEd
Ex, etc.)
June Perry on fri 22 sep 00
The first things we always check for crawling are the cleanliness of the
ware, clay content and thickeness of glaze application. and the dryness of
the glazed pot.
You don't give you glaze formula. Does it contain gerstley borate? If it
does, reformulating the recipe for frit might help. Even a frit or feldspar
can have enough solubility from the glaze standing for a even a few days, to
cause the release of soluble salts on the ware to cause crawling. This is
caused by presumably, the formation of electrolytes being liberated by the
solvent action of the water. This nature or chemical change in the glaze is
hastened by warmth and the possible resultant bacterial action. Totally
drying the glaze and remixing and using right away can correct this. Parmalee
also mention the correction of the PH of such a glaze with the use of acetic
or hydrochloric acid.
He also says that too high or too low bisque firing can contribute to
crawling, (too low and pot takes on more glaze which develops shrinkage
cracks. Too high a bisque can prevent proper adhesion unless the type and
amount of clay is altered or unless adhesives are used).
Pots too closely packed in the glaze firing can also be a contributing cause
of crawling. Glazes sweated in a moisture laden atmosphere during the
watersmoking period, particularly if the flues are wet, will absorb that
moisture, swell and contract as the kiln heats up. Making sure the pots are
fully dry, firing slowly in the early stages of the firing will prevent the
moisture in the glaze coat from being driven out too fast, which would weaken
the layer as well as its bond with the claybody.
He notes that if a claybody isn't properly prepared, it will lack homogeneity
and can cause problems whether the pot is glazed in the raw or bisqued state.
If you have only recently been having problems with this glaze combination,
it could be a change in how one or more of your clay bodies are being mixed.
Crawling can also be caused by excessive grinding, ball milling, which can
increase the amount of soluble salts from a spar or frit. The over grinding
can cause hydration of some parts of the material, causing water to release
during the firing and may also contribut to pinholes and blisters as well.
He goes on to say that the fundamental cause of some of these crawling
causes, as listed above, is the inadequate adjustment of the interfactial
tension of glaze to the body where the crawling appears. To this regard, he
states that much attention needs to be paid to the importance of surface
tension and the viscosity of the glaze melt.
If the glaze is working for you when you get the thickness just right, then
you might want to run a series of tests with different viscosities till you
find what works.
To sum up Parmalee states: The fundamental physical ,singly or collectively,
of crawling and related defects are:
Shrinkages - drying and firing
Adhesion (interfacial tension) or its absence in the unfired state.
Interfactial reaction in the fired stage.
Viscosity of the maturing glaze.
Surface tension of the maturing glaze.
Contributory causes are:
The chemical composition of the glaze
Physical condition of the glaze - fineness of grind, use of frit
Physical conditin of the body - shrinkage, texture(too many non plastic
ingredients, i.e. grog and other coarse materials), vitrification.
Firing duration, temperature, atmosphere.
Thickness of application.
Another consideration would be timing of the second application; but I gather
that since it worked before, you already know that.
He covers quite a bit, more than my fingers feel like typing at 8am. :-( Hope
some of this can help you with the solving problem. I'm sure you will be
getting a great deal more help with this from our resident glaze gurus.
Regards,
June
Cindy Strnad on fri 22 sep 00
Marni,
I suspect the problem is with your underglazes, not with your main glaze. If
you're purchasing pre-made, the manufacturer may have changed the formula,
or if you're making your own, it's possible a change in the ingredients you
purchase has caused the problem. You might try adding more frit to your
underglazes, or thinning them more. The heavy concentration of oxides in
under-glazes tends to cause this crawling effect.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
www.earthenvesselssd.com
Chris Schafale on fri 22 sep 00
Jonathan,
Would you mind giving the names of the ball clays you mention?
Chris
>
> Or rather, use a ball clay that is specifically blended as a glaze clay.
> These are specifically formulated and blended to provide excellent
> suspension characterisitics as well as contribute alumina and silica to the
> mix, obviously. Specifically, Old Hickory Clay Company makes a few ball
> clays that will probably help rectify the problem. There is some wisdom to
> eliminate the EPK altogether, as EPK is not really a good glaze clay and
> substitute one of these very clean burning specifically formulated ball
> clays instead. I think you will appreciate the results.
>
Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)
candle@intrex.net
http://www.lightonecandle.com
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