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Min
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Posts posted by Min
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10 hours ago, davidh4976 said:
4-to-14% EPK depending on the recipe.
If you do decide to work on the suspension I would suggest that replacing some or all of the EPK with OM4 ball clay would help. Given there is less alumina and more silica in the ball clay that leaves room to increase the clay, provided there is silica added to the recipe that in turn would need to be reduced. Just a thought, but on the other hand if your bentonite works then that’s good too.
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Could also try adding a bit of CMC gum solution to the glaze.
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3 hours ago, elenab said:
If the numbers of stabilizers and glass formers are not in any relation with fluxes, does it mean that any recipe with fluxes mol. e. equaling 1 would be good?
Nope again.
Any glaze calc, whether done by hand or with software does a Unity Molecular Formula (UMF) with the fluxes totalling 1.0 when looking at a formula in "unity". (you can also look at recipes in RO, R2O3 or non unity)
If you think of this in terms of baking it might be simpler. Say you are making really basic cookies and flour is the stabilizer (alumina in glazes), sugar is the glass former (silica and boron in glazes) and baking powder is the flux (calcium, magnesium, zinc etc in glazes). Say the total weight of the ingredients is 100 grams. Now if you mixed 1 gram of flour + 98 grams of sugar + 1 gram of baking powder its not going to make a decent cookie. Making glazes is very much about ratio's just as making cookies is. You need to have the appropriate ratio of ingredients to make the type of cookie or glaze you are looking for.
The ratio of fluxes to silica and alumina is very important and is partially dependent on the type of glaze you are looking for. Macro Crystalline glazes for example have very little alumina (stabilizer) in them. High gloss durable glazes should have high levels of both alumina and silica. Many matte glazes have high alumina in relationship to the silica level. Boron comes into play in low and midrange glazes so looking at the silica boron to alumina ratio can also be helpful.
Circling back to your question about how do you know how much silica and alumina should be in a glaze this is where targets can be useful. Glazy dropped using limit formulas in favour of using the Stull chart however it's good to have an idea of what targets to aim for or to be able to look at a glaze UMF and see straightway that a glaze is or isn't likely to be what your are seeking by this information. Test tiles are your best source of information but glaze calc can narrow down the field of possible glazes worth testing. Have a look at some glazes you have used that you like and note the silica and alumina levels plus the type of oxides used for the fluxes. After a while you will start to see patterns.
Screenshot below taken from this link with typical cone 6 target ranges. It isn't a be-all and end-all situation though, many of the more interesting glazes don't fall within these targets or limits but it's another tool in the toolbox for looking at glaze chem.
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The only way around this is to add 2-4% red iron oxide to the clear base to tint it amber which will look more like the dark clay. Problem with this is it will also appear amber on your white lay and in some glazes the iron produces specks in some base glazes.
Part of the problem is from the calcium in the glaze, it’s in most, try avoid high calcium glazes. When I looked up the MC6Glazes liner it is quite high in calcium at 0.64 molar.
- Kelly in AK, Rae Reich and tkw954
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The only thing that might make a difference is the slight increase in silica but my hunch is it won't be significant enough to make a difference, testing will tell.
Given there is no added standalone silica in the recipe to reduce it and get closer to an identical UMF you would need to reduce some of the silica while keeping the other oxides balanced. This is what the Target and Solve function did with it, reduced the silica by reducing the Nepheline Syenite then increased the Minspar (it has less silica than Nepheline Syenite) to rebalance the Sodium and Potassium then adjusted the Ball Clay to adjust the alumina and rebalance the silica.
As well as how the glaze behaves in the bucket particle size and how well they melt comes into play also.
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8 minutes ago, LindsayM said:
That is very interesting. I do seem to be getting bloating bubbly things on the reclaimed clay. I'll try wedging it for longer.
IMHO that really does looks like a bloat/bleb from an air bubble in the clay wall. When the glaze melts it seals the clay over so the gas can't escape and you get those bloats/blebs.
After wedging your usual way try slicing a few chunks of clay up into thin slices and look for air pockets. Keep doing this until you can wedge without getting them. Try cut and slam wedging also if you don't already do this.
I'ld also suggest doing some absorption tests on your claybody given it is being fired below it's maturity, how to that here if you need it, about 2/3 the way down.
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9 hours ago, LindsayM said:
Scarva Earthstone ES10 Extra Smooth.
Given this is a smooth white claybody that fires up to 1280C it could very well be the reclaimed clay just wasn't wedged properly. With pyroplastic claybodies if you don't get all the air bubbles out the clay what can happen during glaze firings is the air pockets expand and cause a bloat (sometimes called a bleb). Are you getting the problem more often with the recycled clay?
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2 hours ago, PeterH said:
... possibly defloculate
Yeah, I'm thinking in this case that might not work as well for the OP here as they are looking to speed up the drying time and defloc's slow it down.
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1 hour ago, Russ said:
Well thats a good question.. probably 1 to 3%. I mix the cmc in a gallon jug with water and let it set a few days. Then add a quart or two to about 4 gallons of glaze when mixing. In this amount it gives even the most powdery glaze a nice hard coat when dry.
Agree with you that gum(s) are great for hardening powdery raw glazes and with some glazes that just don't want to layer up without lifting. I'ld hazard a guess there is much more gum in a commercial brushing glaze than what would be in a dipping glaze to prevent those issues though.
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17 minutes ago, Russ said:
I use cmc gum in all my glaze formulas and I dip and spray
What percentage?
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15 hours ago, moh said:
Anyone know how to counter the effects of gum in commercial glazes?
Try freezing a test amount of it. If it works you will probably need to increase the specific gravity of the glaze for spraying it without overwetting the pot.
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@davidh4976, I was thinking more about the 450F temp and crazing. Is the ASTM554 test still valid if the body cracks but the glaze doesn't?
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16 hours ago, neilestrick said:
Here's the really interesting thing, though: in all cases, the unglazed areas were a lot hotter than the glazed areas. When I measured down the side of the piece, they were all cooler at the top than they were at the bottom, presumably because they all have unglazed bottoms.
My guess would be it's because of different reflected indexes between the glazed and unglazed surfaces while in the microwave.
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1 hour ago, davidh4976 said:
Maybe you mean ASTM 554? It calls for three passes of going from 250F to room temperature water without crazing. Repeat at +25F increments up to 450F. Crazing fails the test. Too much work for me! I just rely on three passes of boiling water to ice water to determine whether it's going to craze or not. It is a little bit more of a cheat for me because boiling water at my altitude is only 199F. Although the microwave test calls for doing ASTM 554 first, to me, the two tests seem mostly independent. If you are going to determine whether a piece is microwave safe, microwaving it and seeing if it gets too hot seems to be enough for me.
I question the testing using the same piece for each subsequent heat increment as each cycle will stress/shock the glaze. I know that's the point but I agree with Hansen's take on this. I sometimes go 300F/ice water, then 315F/ice water then 325F/ice water. If a glaze doesn't stress craze from this I don't think it will with everyday normal (sane) usage. 450F to ice water IMHO is far too extreme! We are after all making ceramics, not metal bakeware. It does seem logical to soak the test piece for 12 hours after a reasonable stress/shock testing though.
edit: In the past I did try the boiling water / ice water testing, had a glaze that passed it so went ahead and glazed a load of pots with it which later went on to craze. I'm sticking with the oven/ice water test, greater difference in temp therefore a harsher test, either the one above or just 3 sets of samples at 315F.
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Without going into too specific details to follow the ASTM... for cups and mugs put a cup of cool water in the corner of a microwave then with the empty test sample mug run it for 2 minutes at full power. Handle temp must not exceed 60C/140F and any of the other surfaces 121C/250F. Before commencing test the test pieces must be submerged in room temp water for 12 hours and also pass the ASTM 325F oven to water test. (which I don't have) Slightly different times and temps for plates etc.
- Kelly in AK, neilestrick and Hulk
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7 hours ago, brooksms said:
Does anyone know how much should be added
I wouldn’t use either of those glazes if you are new to glaze chemistry as it would be necessary to remove some of the other fluxes in order to add zinc. If you just added zinc then the glaze would have more flux than needed to create a balanced glaze. Result would be an excessively fluid glaze.
Glazes high in magnesium tend to have a more muted response to colourants than those without high levels of it and blues from cobalt lean more towards purple tones.
Yes, zircopax will give more of an opaque glaze, with or without stains. Starting at around 5% addition will show, 10% will be opaque on most bodies.
A green stain that contains chrome when put in a glaze containing zinc will most often give an unpleasant brown.
Your second link didn’t work for me.
Welcome to the forum.
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Neoprene disc stuck to the wheelhead with a little water for the vast majority of my pots, no clay wads. For bowls and platters wider than the wheelhead I use a large batt that I glued high density upholstery foam to and put that on a damp Xiem BatMate knockoff. Never felt the need for a Giffin Grip.
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In a nutshell to pass the ASTM for "Microwave Safe for Reheating" there is mention of "no metallic surfaces" but no threshold limit for porosity or absorption of the claybody. Ware must show no degradation after the test nor exceed specific temperatures after a set amount of time at full power.
I would hazard a guess that those discoloured areas on the plate have let moisture into the body which resulted in the plate getting hot when your son removed it from the microwave. Agree with you, that is one well fitting glaze to stand up for catering use then your home use for 20 years.
- Kelly in AK, Rae Reich and PeterH
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You can for sure use wadding in an electric kiln to raise pots off a shelf or level shelves but I wouldn't use it on a glazed surface.
If you stack plates with wadding between unglazed areas on the plates chances are the plates will warp and deform at mid or highfire.
I don't know the price of setters compared to kiln shelves where you are but another option is to purchase small round kiln shelves and use those with short posts for setters. If you try this be careful to line up posts to prevent cracking a kiln shelf when you stack them, make sure the posts line up.
4 hours ago, Tiny Island Design said:why, since the wadding does not stick to items, then why does it leave marks?
When wadding is used in an atmospheric firing such as wood, salt or soda the vapours from the gasses in the kiln flash the pots but the areas where wadding is will resist the flashing.
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Laguna has a chart showing all their claybodies and what they are recommended for, including throwing and handbuilding large.
https://www.lagunaclay.com/_files/ugd/e5330f_25040094ad64486892fa46e3a38e608f.pdf
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One to look at might be this one for the blue, using the pot spar and cobalt carb you have (increase the cobalt carb to approx 1.60) for the blue glaze, not many ingredients in it and it has a good history. I'ld give it a try with the grolleg you already have. I would also try it with an increase the red iron oxide to tone down the blue a bit and maybe 1 - 1 1/2% manganese dioxide if you have some. For the rim and foot I'ld start with a test of a simple 50:50 mix of your wood ash + your local red clay and see what it does.
I agree with Mark, the pot you posted looks more like a salt fired pot rather than soda.
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questions about UMF
in Clay and Glaze Chemistry
Posted · Edited by Min
@elenab, the chart you posted in your post above isn't for glazes, it's the formulas for making cones.