Ugh! This seems to happen to one piece in every firing (cone 6). There is a white crusty blemish on the glaze that is dry (the glaze is a gloss white glaze) and almost looks like flashing (but is not). I think it is scumming from what I've read, and that it might be caused by soluble salts in the water. I am wondering if they might be in the water I mix up the glaze with, since the problem doesn't seem to be on the clay, and it has happened on different clay bodies that had the same white gloss glaze. I think I read that this means the water I use is too "hard". Question: might it fix the problem if I used water from a different source the next time I make up my glaze recipe (distilled, rain water, etc)?
Possible Scumming On Glaze?need to remedy this problem!
Posted 15 June 2014 - 06:30 PM
It sounds like mixing your next glazes with distilled water would probably be the easiest troubleshooting step.
Posted 15 June 2014 - 08:20 PM
Posted 16 June 2014 - 07:17 AM
Thanks, Babs, for that link, but it seems to be related more to scumming on the clay body rather than on the glaze, which is what I think I'm experiencing. Bciskpottery - The glaze recipe does actually have tin oxide in it. Why/how would the crystals effect the problems I'm having, if they are forming in the glaze bucket? Is there a way to get rid of the crystals? How are they related to tin oxide?
Posted 16 June 2014 - 10:59 AM
Could you post a picture so we are all on the same page as to what you are calling scumming?
Posted 16 June 2014 - 05:54 PM
White body here no apparent scumming until it shows up on one of my glazes. Don't know about scumming in glazes.. underfired? unbalanced formula??
Posted 16 June 2014 - 08:52 PM
Here is a picture of the blemish. It is hard to see because everything is white. Could it be a problem caused by tin oxide?
Posted 17 June 2014 - 06:16 AM
Can't wipe it off. It is hard and crusty. I tried sanding it off and just got to bare clay.
Posted 17 June 2014 - 12:10 PM
Post the recipe.
Too close to the kiln element there?
Kiln Repair Tech
Owner, Neil Estrick Gallery, LLC
Posted 17 June 2014 - 08:21 PM
White Glaze Recipe: (cone 5-6)
Gerstley Berate 11.3
Texas Talc 13.9
Neph Sy. 39.1
add: tin oxide 8
Definitely not too close to the kiln element as the latest victim was in the center of the shelf. However, it was close to a stilt, although I can't quite see how that would effect it. Not sure if the blemish was exactly where the stilt was, since I picked the bowl up and turned it around a few times before I saw the dreaded spot....
Posted 17 June 2014 - 09:57 PM
I ran your glaze through Insight and it looks fine. Silica to Alumina levels are fine, magnesium is high but shouldn't cause this problem.
Your comment about being able to sand the glaze down to the clay makes me think the glaze hasn't bonded with the clay. I don't know if soluble salts would cause that, my thought would be that the salts would cause more fluxing but I'm not sure. Looks like a frustrating problem, I have never seen a glaze do that.
Gerstley Borate............. 11.30
Nepheline Syenite........... 39.10
EP Kaolin................... 9.50
CaO 0.25* 5.15 5.71
MgO 0.48* 7.10 10.96
K2O 0.06* 2.07 1.36
Na2O 0.21* 4.72 4.74
P2O5 0.00* 0.04 0.02
TiO2 0.00 0.05 0.04
Al2O3 0.38 14.05 8.57
B2O3 0.13 3.33 2.98
SiO2 2.88 63.32 65.55
Fe2O3 0.00 0.18 0.07
Calculated LOI: 9.10
Thermal Expansion: 6.69
Formula Weight: 273.06
Posted 18 June 2014 - 02:20 PM
Dipping to glaze. It is abrasive, but not sharp - just rough....
Posted 18 June 2014 - 02:27 PM
Are you using witness cones? I'm wondering if it's underfired. Does your kiln fire evenly?
White glazes are usually about the easiest to make, have you tried another white glaze, might be easier than trying to fix this one if all the other variables are ruled out.
Posted 18 June 2014 - 02:59 PM
It is a bit high in Tin oxide. I would think you could drop it down to 5-6% and still get it plenty opaque. And with the price of tin, it wouldn't hurt to try using less.
Fire with some cones to verify it's getting to temp.
Kiln Repair Tech
Owner, Neil Estrick Gallery, LLC
Posted 18 June 2014 - 08:17 PM
If your dipping, is it poss that you have not stirred enough and that this section is getting into a thick spot towards the bottom of you glaze bucket and thus loading up with some of the less melty stuff in the glaze?
Is Gerstley playing any tricks here Neil?
Posted 18 June 2014 - 08:39 PM
I'll try dropping the tin oxide level down a bit (yes, very pricey!). I am loath to use another white glaze since this recipe works really well on my dark brown clay body that is fussy with white glazes, as well as with this white body. I fire with cones and am sure it is getting to temperature because the rest of the ware on all the pieces have a nice melt. I've under fired this glaze before and it goes matte, but not like these weird little dry, bright-white almost star-burst shaped spots against the glossy surface when fired to temp. Usually there is only one per kiln load. It seems to be quite a mystery. Doesn't sound like summing after all. Oh well, maybe I'll have to go back testing white glazes. Not great timing as I have some orders I need fill and don't really have time to start from scratch. Boo boo!
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