Potters & Sculptors - Making Rock from Mud
I would appreciate some feedback re this Jens Juicy fruit glaze I tested the other day. I fired a kiln load of tests with success EXCEPT for this one glaze.
Recipe I used
Neph Syenite 49
Gerstley Borate 9
Red Iron Oxide 4.0
Soda Ash 10.0
It has blistered where thicker (in the deeper texture lines you see and underside of the test ring) and I can pop the blisters to expose the perfect glaze underneath. I had one on the top shelf and one on the bottom which is usually cooler with the same outcome.
I fired the kiln to a flat cone 6, as i soaked at just under 1200 for about 1/2 hr. Kiln naturally cooled to 900c, at which time I turned back on a held for about 1 hr.
Could the blisters be due to overfiring or perhaps due to thickness of applicatoin. Just confusing as other that the areas of blisters, the glaze looks great and how I would expect it to turn out?
Thanks in advance. Jan
I'm trying to trouble shoot the same recipe. I don't have blistering, but I have some crawling and a lot of pin holes and small crater like holes.
The most common reason for blistering is an overly thick application of glaze. Firing too hot or too quickly at the end of the cycle can also contribute to glazes blistering. I would try soaking it at the top temperature for a half hour before cooling to resolve the blistering.
I've also read that soda ash can produce large amounts of out-gassing.
The next time I use this glaze, I am going to try soaking it at 1800F to see if I can develop some more color. It came out very dull and muddy on my test tile, but I tried it on a pot anyway with some interesting results. I like it except for the trouble spots. I'll let you know how that goes.
Hi Jen, Thanks for your comments. I agree with what you have said and I think that for my results above, my glaze application was a little too thick. I have come to this conclusion as the glaze only blistered in the deeper lines on the tile where the glaze was thicker as the thinner area came out fine where. Perhaps your glaze results are a result of application of glaze and thickness too? The trials and tribulations of glazing. Sometimes very frustrating and other times a joy.
John Post has very interesting remarks about firing Jen's...
Also one may question the accuracy of the pyrometer reading. I have used 2 in one firing as insurance.
Several glazes use soda ash, and I'm not sure why.
The following is copied from Tony Hansen's Ceramic Materials Database:
The most common form of sodium carbonate
Formula: Na2CO3 or Na2O.CO2
DENS - Density (Specific Gravity) 1.44
MLPT - Melting Point (MP) 34C D
In ceramics, a common use of soda ash is as a soluble deflocculant in ceramic slips and glazes. It works well in combination with sodium silicate to produce slips that do not gel too quickly and whose rheology can be adjusted for changes in the hardness of the water. Higher soda ash in proportion to sodium silicate will produce a slip that gives a softer cast (stays wet longer). The total soda ash and sodium silicate amount should be tuned to create a slip that will eventually gel if left to stand. This thixotropic behavior will prevent it from settling.
Sodium carbonate is the preferred deflocculant for thinning glaze slurries.
Soda ash is not normally used as a source of Na2O in glazes because it is soluble. It is used as a source of sodium in frits and glass. Its solubility makes it an ideal flux for Egyptian paste glazes."
I speculate that the blistering is CO2 from the soda ash that absorbed into the clay body. As the temperature goes up the soda ash breaks down into Na2O and releases CO2 gas that builds up and tries to escape to the atmosphere, creating bubbles if trapped in the liquid phase glaze. Perhaps the blistering is an unwanted side effect of trying make to the glaze slurry easier to store or to apply?
It would be interesting to create an equivalent glaze sourcing Na2O from non water soluble material.
I have a same problem drive me insane. a lots of place saying that temp should be high and hold a while fro soak. do you have any idea for firing schedule?
I seem to recall this same problem when using it by itself. When layered with other glaze, it usually behaved. I had 2 different formulations. One was from Glazemixer & the other one I mixed myself. I don't know if they were identical, but they both had issues. When I do some more Stephen Hill glazing, I think I will try Norms revision & see how it behaves. When I used it in ^10R at the art center, there were less problems. jhp
May I suggest Jen's Juicy Fruit - Revised.
I've never been happy with the original, so I've made a version without soluble ingredients.
The Insight chemistry looks somewhat different only because Digitalfire does not use the current chemistry for Gertley Borate. I could remake my version to match the old version if you had used old gertley borate.
My only problem so far has been crazing, everything I've glazed with it so far has crazed.
This mug is probably the worst example and the other pot is the least crazed, it's hard to see it, but it is crazed.
Edit:a misunderstanding with the image posting - but they are both there eventually - albeit in two posts. ;)