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Natania

Possible Scumming On Glaze?

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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)?

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Is it always the same glaze? I've had that happen on some of my pieces, and was told that it was because the kiln fired too low (our studio kiln needed replacing badly and that load was the tipping point!)

It sounds like mixing your next glazes with distilled water would probably be the easiest troubleshooting step.

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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?

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White Glaze Recipe: (cone 5-6)

 

Dolomite.                   7.2

Gerstley Berate        11.3

Texas Talc                13.9

Neph Sy.                   39.1

EPK                            9.5

Silica                         19

 

add: tin oxide              8

        bentonite.           2

 

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....

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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. 

 

 

Dolomite.................... 7.20
Gerstley Borate............. 11.30
Talc........................ 13.90
Nepheline Syenite........... 39.10
EP Kaolin................... 9.50
Silica...................... 19.00
=========
100.00

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

Cost: 0.37
Calculated LOI: 9.10
Imposed LOI:
Si:Al: 7.65
SiB:Al: 8.00
Thermal Expansion: 6.69
Formula Weight: 273.06

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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. 

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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?

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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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Is there any ingredient in the glaze (white) that could be forming crystals in the glaze bucket (e.g., tin oxide)? and have you eliminated flakes of kiln wash as a source?

How is this, although the problem appear in one or two pieces ??

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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.

 

 

Dolomite.................... 7.20

Gerstley Borate............. 11.30

Talc........................ 13.90

Nepheline Syenite........... 39.10

EP Kaolin................... 9.50

Silica...................... 19.00

=========

100.00

 

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

 

Cost: 0.37

Calculated LOI: 9.10

Imposed LOI:

Si:Al: 7.65

SiB:Al: 8.00

Thermal Expansion: 6.69

Formula Weight: 273.06

Dear Mr. Min

if I have a new glaze recipe , and I want to know at what temperature it will fired Theoretically before putting in the kiln ???????

 

Regards

Ashraf Elhamy

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