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Making A Glaze More Runny


Saki

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I recently ran tests with Cone 6 Ultraclear Glaze for Porcelains and was pleased with the surface texture (glossy) and color results I obtained using copper, cobalt, iron and rutile oxides. However, when overlapping glazes, the border between the two glazes is very crisp. I would like the top glaze to drip down over the bottom glaze, so that the border between them is more fluid. Is there a way to make this glaze more runny? I thought about adding Lithium Carbonate because I have read that it is a powerful flux. But I wonder if there is a better way?

 

In case it is helpful, I have attached some photos of pieces with the runny, drip-glaze overlap effect that I am looking for.

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Well.... two glazes of the same base generally don't have "reason" to chemically mix. This means they generally won't feather into each other. I think the transition you want is achieved with two different base recipes.

 

Adding more flux would make the glazes more fluid but if they still have the same base I cannot see it happening as planned. More likely the glaze would flow onto the shelf! Obviously you are free try and I am only guessing the outcome.

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Well.... two glazes of the same base generally don't have "reason" to chemically mix. This means they generally won't feather into each other. I think the transition you want is achieved with two different base recipes.

 

Adding more flux would make the glazes more fluid but if they still have the same base I cannot see it happening as planned. More likely the glaze would flow onto the shelf! Obviously you are free try and I am only guessing the outcome.

That's a good point, and it is precisely the reason I am looking to alter the top glaze. Since the glaze works in other respects, I thought it might be preferable to adjust this recipe, rather than start with another. This approach seemed more systematic and controllable. But if there is a better method, please let me know. I am open to suggestions!

 

Besides fluxes, are there other materials that generally interact this way when overlapped?

 

As far as glaze running off the shelf, I think this might be avoided by application of the top glaze to the top of the pot (the exact limit would require additional testing).

 

Also, it's interesting to me that the the turquoise glaze in the 3rd photo seems to have achieved a runny effect without overlapping on another glaze. I would love to achieve that effect.

 

Thanks

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Do not know what cone you are firing to, so lets say ^6 for now.

 

Looking at your pictures: I smell Whiting in your glaze recipe. At ^6, 18% or below Whiting (calcium) will not run, although it would be fluid. 20-24% will begin to run, and 26-30% will become a river. At 2232F, potassium and sodium are gases: when these two are over fluxed they show up as pinholes and craters. Lithium, calcium, magnesium, and strontium are liquid at 2232F. When any of these are over fluxed: they will run like a river. So I will hedge my bets on Whiting (calcium) being the flux used to produce the effects shown.

 

However, when you cross the 20% line on calcium, you also cross the line on formula limits. One of the reasons calcium is set at 0.60 max: because it begins to run. Likewise potassium and sodium at set at 0.30 limits: because pin holing begins after that. Does not mean you cannot cross the 0.60% line with calcium: just means the glaze will begin to run. In your case, a desirable effect. Same thing occurs with intentional offset COE values to produce a crackle glaze. Run it up over 0.60 to get your desired effect, but come back and set formula unity as much as possible.

 

Nerd

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Nerd,

 

Thank you so much! This is just the kind of information I was hoping for! The recipe uses Wollastonite, rather than Whiting, but my understanding is that both are sources of calcium, so I'm not sure if it makes a difference. Here is the full recipe:

Material———Amount

Minspar 200———8.6

Frit 3134———23.2

Wollastonite ———15.2

EPK ———24.8

Talc ———4.3

Silica ———23.8

Total———99.90

 

And here are the Unity Formula values (calculated using GlazeMaster software):

Na2O–––0.140

K2O–––0.044

MgO–––0.128

CaO –––0.688

 

Al2O –––0.367

B2O3–––0.258

Fe2O3–––0.005

 

SiO2 –––3.451

TiO2 –––0.003

Ps05 –––0.001

 

Si:Al –––9.40

COE –––382.15

LOI–––4.20

 

According to the notes for the recipe, the COE is on the low side because this is a better fit for porcelain.

 
The Calcium is already over 0.6, but if I understand correctly increasing it will help make the glaze more fluid. Assuming that's right, do you think the following revisions might be a good place to start?
 

Material———Amount

Minspar 200——— 8.0

Frit 3134———21.8

Wollastonite ———24.5

EPK ———23.3

Talc ———0.0

Silica ———22.4

Total———100.0

 

Unity

Na2O–––0.88

K2O–––0.0.48

MgO–––0.0.34

CaO –––0.831

 

Al2O –––0.435

B2O3–––0.148

Fe2O3–––0.006

 

SiO2 –––3.634

TiO2 –––0.003

Ps05 –––0.001

 

Si:Al –––8.36

COE –––386.55

LOI–––3.93

 

You mentioned formula limits, but I am not sure where to find these. Are these fairly standard? Or would they be different for matte glazes and glossy glazes, for example?

 

Also, just to clarify (in case I was confusing), the photos are examples of the drip effect I am looking for, but they were not made with this glaze.  

 

Thank you again!

Saki

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This topic came up a few months ago and Paul Lewing seemed to have the best answer,

"Posted 25 March 2016 - 08:41 AM

The solutions to your problem that have been proposed have all been process or physical fixes, but the root of your problem is chemical.  If you want glazes to bleed a long way into each other, they need to be very dissimilar.  Conversely, glazes that are similar will bleed into each other less.  And the key area of difference is Si:Al ratio.  If the two have ratios have difference from each other that is greater than 6 they will definitely bleed a lot.  For instance if one has a Si:/Al ratio of 5 and the other has a ratio of 12, expect a lot of bleeding.  This of course makes sense.  One is deficient in silica, the other is deficient in alumina, so the go looking for what they need in the other.

So... bottom line, if you want no bleeding at all, your best bet is color variations on the same glaze.  Doesn't matter what glaze it is.  The the only variable will be how much of a flux or refractory the colorant is in each different color variation."

 

An easy way to make your G1216M glaze more runny would be to just decrease the alumina, (epk in this recipe), test by running a line blend. It will increase the likelihood of crazing though.

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Saki:

 

I have a big issue with your fix>

The alumina % was 0.367 in the original recipe. It is now 0.435 in your suggested fix. Alumina gives glaze its strength, but it also a high temperature viscosity agent: also used to hinder glaze run. So by increasing your alumina, you have shot yourself in the fluidity. Normally I would be pushing alumina limits: because higher amounts reduce cutlery marks, etc, In your case however, you want fluidity. Si/AL ratios are the heart of any glaze.

The simple fix is to just add whiting.  Whiting is 52% calcium, and wollastonite is 42%. The problem I have with wollastonite is the magnesium (MGO) content. You are crossing into the % level of getting "gold dust" in your glaze: think temmoku.You have boat loads of silica, way more than needed. In this case some of the EPK need to go to get the alumina ratio down. The frit makes it fluid as well, but some loss will not hurt.

 

Minspar           8.6

Frit 3134        21.7

Wollastonite   15.2

EPK                21.8              Total alumina in formula is now 0.293   (low side of limits)

Talc                  4.3

Silica              23.8

Whiting          5.0                Total calcium in formula is now 0.735    ( limit 0.60)

 

Because the alumina has been greatly reduced, and the calcium greatly increased: my guess is your glaze is something like the Mississippi river. Might have over done it- test firing will tell.

 

The other fixes suggested are viable options as well, and should be considered.

 

Min said:   And the key area of difference is Si:Al ratio.

Si/Al is the heart of any glaze. Reason I build that first, formula limits second, and unity third.

 

Nerd

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Nerd,

 

Thank you again for your careful critique.  I am sorry, I am still new to glaze chemistry and hadn't realized that 0.435 was outside the limit for alumina. My recollection was that Roy and Hesselberth's Mastering Cone 6 Glazes recommended somewhere between 0.25 and 0.45 for alumina and between 3.0–4.0 for silica, but it is quite possible my memory is faulty. I will give your revised recipe a try.

 

On a related note, are there any books or online resources you might recommend for learning more about these limits?  your point about magnesium causing the gold dust effect of tenmoku is fascinating. 

 

Thank you,

Saki

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Saki: actually I need to apologize: my wording gave you a false impression.

 

 

and hadn't realized that 0.435 was outside the limit for alumina.

Alumina formula limits are 0.25 to 0.50 (Roy & Hesselberth.)  I was informing you that raising the alumina will have the opposite effect of hindering glaze flow. For this same reason glazes with different SI/AL ratios will bleed into each other. It has less to do with silica as it does alumina. A glaze with low alumina will run more, and into a glaze with high alumina until it leaches enough alumina to slow it down/stop the run. That is partly how they produce those pictures you show above.

 

Nerd

 

Sorry that I cannot recommend any books: seeing as though I read very few of them. Most do not have the technical information I am looking for. Helmer and Helmer is a must have resource however. Guess I should fess up: I talk to Ron Roy on a monthly basis in email, I would have to call him my teacher at this point.

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Nerd,

 

Ah, now I see. Your wording was clear; I just misread it. Based on this information, I think I will try a few different tests: (1) the glaze with whiting and with lower Alumina and (2) combinations with another glaze with higher Si/Al ratios.

 

I am not familiar with the Helmer and Helmer resource you mentioned and a search for "Helmer and Helmer" on Amazon didn't turn up anything. Could you let me know what that is and where to find it?

 

Thanks for your continued help and generosity sharing your knowledge!

 

Saki

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Just about to say that myself Fred :D Interesting book, look for second hand books as I bought one for 1/10th of the new price.

 

https://www.amazon.co.uk/Potters-Dictionary-Frank-Hamer/dp/0713664088

 

I have not seen high calcium make a glaze very fluid except in weird situations where something crystallizes out my glaze and then makes it very fluid in that area. Still dropping the alumina will get you closer and increasing the calcium may help it along.

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TY Fred- yes Hamer & Hamer. I had just got done doing some research on Helmer clay....oh joy!

 

Saki, a picture is worth a thousand words- so this old glaze run test illustrates my point. You can use alumina limits to control glaze flow/run.

 

 

 

The left tile has alumina at the top of alumina limits, and the right tile has alumina at the bottom of formula limits. So in addition to increasing calcium limits to produce glaze flow, you likewise lower alumina to further that goal. An additional point I forgot to cover: application rate/s. Typically glaze is applied at 0.15 grams per square inch or so. Drives me crazy when potters say: "two second dip." What the h''l kind of data is that? I apply crystalline or cone 6 standard by grams per inch. The glaze run test shown is applied at 0.65 grams per square inch. (GPI). So how much glaze you load at the top will also determine how much will run, and how far.

 

Nerd

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If you have Hamer and Hamer in hand, have a look at the sections on Viscosity and Surface Tension. I think you will find them helpful in looking at this issue.

 

In particular look at the tables in those sections that rank different materials' viscosity and surface tension by different temperatures. Stare at them until their meaning and order becomes clear. There are some interesting results.

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Thank you, Curt! Those sections were extremely informative, particularly the table of oxide influence upon viscosity and accompanying notes. It was interesting to me that there is generally an overlap between oxides that lower viscosity and lower surface tension; however, not in every case. For example, Calcia and Magnesia lower viscosity (it is in Zone B), but increase surface tension. This makes me think that adding/increasing oxides like lithia, soda, and potash might have a more pronounced effect.

 

These section also led me to the Periodic Table section, which has an extremely detailed overview of chemistry for the potter that I found immensely helpful, too.  

 

Thank you again for pointing me to these resources.

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  • 4 weeks later...

TY Fred- yes Hamer & Hamer. I had just got done doing some research on Helmer clay....oh joy!

 

Saki, a picture is worth a thousand words- so this old glaze run test illustrates my point. You can use alumina limits to control glaze flow/run.

 

 

 

 

The left tile has alumina at the top of alumina limits, and the right tile has alumina at the bottom of formula limits. So in addition to increasing calcium limits to produce glaze flow, you likewise lower alumina to further that goal. An additional point I forgot to cover: application rate/s. Typically glaze is applied at 0.15 grams per square inch or so. Drives me crazy when potters say: "two second dip." What the h''l kind of data is that? I apply crystalline or cone 6 standard by grams per inch. The glaze run test shown is applied at 0.65 grams per square inch. (GPI). So how much glaze you load at the top will also determine how much will run, and how far.

 

Nerd

How do you measure  grams per square inch? 

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Hi Janice:

 

Somehow your question got by me unnoticed; my apologies. Crystalline glaze requires a minimum glaze application rate in order for crystals to grow. So I measure a tile 10 x 10 =100 x .50Grams per square inch = 50 grams. So I mix and spray 50 grams on one tile. I know the "two second" dip rule is very popular with standard glazes. I am not use to dealing with glaze in that fashion. So I weighed a tile before glazing: dipped for two second and reweighed= .15-.20 grams per SI.

 

Nerd

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I happened across this clayart thread on layering glazes

http://www.potters.org/subject12168.htm

 

I attention was particularly taken by this item by Paul Lewing:

 

It's a basic question, but not a simple one to answer. A lot of it has to
do with a principle called eutectics, which basically says that the
combination of any two fluxes melts at a lower temperature than either one
separately. So you'll often notice that if you put two different color
variations of the same base over each other, you get very little of this
"action". The more different the two glazes are from each other, the more
they interact. You will also see that if one has a high alumina-to-silica
ratio, and the other a low ratio, you will get more interaction. One other
thing that affects the patterns you get is the amount of outgassing the two
glazes do. For instance, say you're layering a copper red and a temmoku in
reduction. The temmoku will bubble and boil a lot more than the copper red,
because there's so much iron being reduced. So if the temmoku is
underneath, you'll get a more broken-up pattern than if you layer them the
other way.
But there is never any way to predict what will happen exactly when you
layer glazes. It's too dependent on too many variables- relative thickness,
the order they're applied, how much reduction or oxidation, the temperature,
the firing cycle, the pitch of the wall they're on, the chemistry of the two
glazes, and so on. You just have to do it and see what you get.

 

... which raises the possibility that the "mixed alkali" effect can also be involved.

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