Verifying glaze results

[thiamant]

Hello!

How can I check if a matte glaze is melted or underfired? The stull diagram looks ok but this doesnt take temperature into account.

[Bill Kielb]

You can fire it progressively higher. A true matte will become a runny matte. Really the only way to tell is to test fire in a range to get the best melt. There are no simple predictors that will tell you what cone a composition will fire to, it needs to be tested. Often you will see folks say this test fires well between cone 5 and 7 or some set of numbers they tested it at.

[Magnolia Mud Research]

1 hour ago, Bill Kielb said:

There are no simple predictors that will tell you what cone a composition will fire to

Bill, 
are there any non-simple predictors? 
 

LT

[liambesaw]

3 minutes ago, Magnolia Mud Research said:

Bill, 
are there any non-simple predictors? 
 

LT

Boron content for low and mid range, but it's not simple because boron doesn't melt alumina.  I think it depends on the si:al ratio and boron content.  And even then it's an estimation depending on other refractory ingredients.  I could be wrong but that's what I've seen.

[Min]

Limit Charts can be a bit of help. If you first look at the silica and alumina amounts then go to boron then to remaining oxides. Some of the fluxing oxides work better at higher temperatures so knowing your materials comes into play too. Problem with limit charts is they don't take into account glazes where one or more oxide needs to be high to get a specific glaze type, calcium mattes for example. 

 

[Bill Kielb]

2 hours ago, Magnolia Mud Research said:

Bill, 
are there any non-simple predictors? 
 

LT

Interesting question but I would  agree with @liambesaw boron as I know it would be about 0.15 expected to melt at cone 6,, 0.45 expected to melt at cone 04 but obviously composition can affect that a whole bunch. Interesting ranges of zinc as in Bristol glazes generally get  expected cone 6. Good question though, what indicators do potters feel glazes melt at cone 6 and so forth?

[liambesaw]

You could look up the melting point of each ingredient and average them based on melting point and weight.

  I dont think there are any common eutectic compositions in common glaze materials, so that should work handily.

 

[Bill Kielb]

14 minutes ago, liambesaw said:

, so that should work handily.

Not sure about that actually. If we look at Bristol glazes 0.2-0,4 zinc in a .7 RO base can get you down to cone 2 actually. Folks who study these probably can be more specific, not sure if it fits the weighted average of melting points. If that worked one would think someone would have simply created a handy spreadsheet.

[Min]

@liambesaw, there is a really neat eutectics calculator available from Robert Magnuson (chemist and potter), there is a link to it in the Feb 2018 CM. I can post what it does here but I can't link the program. He made it for Excel but I just tried it with Mac Numbers, don't know if all the functions work with it as I've just started playing around with it but I did get it to work so far. Theory is that the excess oxides that are not taken into the eutectics melt will precipitate out therefore making a matte glaze with the appropriate cooling (or not in the case of high alumina mattes). I made up a  recipe to purposely overload it with magnesium just to see what it did, screenshot below. You can see the oxides that aren't taken into the eutectics, 0.54 MgO, 0.18 Al2O3 and 1.62 SiO2. Flip this theory around and you could supply only the necessary oxides to make a transparent glaze; in other words you would know how much magnesium or calcium etc is oversupplied and remove it from the UMF.

Not seeing how it could be used to predict firing range though.

[liambesaw]

Hahaha, man was I wrong about eutectics. There are a bunch of them in glazes.  That really throws a wrench into calculating melting points: https://digitalfire.com/material/eutectics

 

[thiamant]

6 hours ago, Min said:

@liambesaw, there is a really neat eutectics calculator available from Robert Magnuson (chemist and potter), there is a link to it in the Feb 2018 CM. I can post what it does here but I can't link the program. He made it for Excel but I just tried it with Mac Numbers, don't know if all the functions work with it as I've just started playing around with it but I did get it to work so far. Theory is that the excess oxides that are not taken into the eutectics melt will precipitate out therefore making a matte glaze with the appropriate cooling (or not in the case of high alumina mattes). I made up a  recipe to purposely overload it with magnesium just to see what it did, screenshot below. You can see the oxides that aren't taken into the eutectics, 0.54 MgO, 0.18 Al2O3 and 1.62 SiO2. Flip this theory around and you could supply only the necessary oxides to make a transparent glaze; in other words you would know how much magnesium or calcium etc is oversupplied and remove it from the UMF.

Not seeing how it could be used to predict firing range though.

How is temperature taken into account in this approach?

For example this is my dolomite matte. I want to check if it melted at cone 9 during my last firing. Its not a runny glaze so it is difficult to evaluate whether its underfired or not.

https://glazy.org/recipes/140261

It looks melted to me:

https://s3.amazonaws.com/glazy.org/public/uploads/recipes/61/l_140261.608531b89aa02.jpg

[Min]

4 hours ago, thiamant said:

How is temperature taken into account in this approach?

 

8 hours ago, Min said:

Not seeing how it could be used to predict firing range though.

@thiamant, Liam brought up eutectics so I mentioned the eutectics calculator. Chasing squirrels.

If your magnesium matte is durable at the cone you are firing it at and your claybody is mature then I don't see a problem. I could see dropping the alumina a little though.

[thiamant]

Ok thank you @Min. I also wanted to turn it more glossy by adding a bit of silica maybe then I can reduce cutlery marks too. but then I would need to drop alumina maybe? Or add more flux?

[C.Banks]

3 hours ago, Min said:

 

@thiamant, Liam brought up eutectics so I mentioned the eutectics calculator. Chasing squirrels.

Some folks trying to herd squirrels over at glazy:

https://glazy.org/posts/166279

https://glazy.org/posts/110706

Eutectics make my head spin a bit

[thiamant]

Oh I see, I am more familiar with Montmollin fuse diagrams because they are implemented in new Glazy calculator:

 

The upper left point is the original glaze recipe, the lower right is 10 less kaolin 10 more silica

 

These Montmollin fuse diagrams were made for cone 10? Are they also useful for cone 6 glazes?

[Min]

2 hours ago, C.Banks said:

Eutectics make my head spin a bit

Maybe a combination of the eutectic calculator plus a UMF chart would be less squirrely to look at? 

1 hour ago, thiamant said:

The upper left point is the original glaze recipe, the lower right is 10 less kaolin 10 more silica

I would try more like cutting the kaolin in half and adding enough silica to bring it back up to the 2.9 or a bit higher, depending on how matte you want the glaze. See where it plots on your chart.

Another way to alter the glaze would be to take a clear gloss glaze (any gloss you know that melts well and fits your clay, skip any colourants or opacifiers) then do a volumetric line blend of the two glazes. Video below from John Britt showing one using the same glaze but in two colours. You would do the same thing with your matte and gloss glaze. There should be somewhere along the line blend where cutlery marking stops and yet the glaze is still matte enough for your purposes. 

https://www.youtube.com/watch?v=1ty81TEAiEk

[C.Banks]

1 hour ago, thiamant said:

These Montmollin fuse diagrams were made for cone 10? Are they also useful for cone 6 glazes?

Daniel De Montmollin's work is only fairly recently getting the credit it deserves. This being said ..."melting diagrams ... are not easy to find and hard to understand for the layman (aka potter)."

https://wiki.glazy.org/t/the-practice-of-stoneware-glazes-d-de-montmollin/860

iirc the chart designed by Ray Thomas Stull was also originally intended for ^11 but does indeed have value for mid range glazes.

The folks working to incorporate the two are waay more technically orientated than me but I imagine there is useful overlaps between stull charts, fuse diagrams and cone 6 limit formulas.

[C.Banks]

26 minutes ago, Min said:

Maybe a combination of the eutectic calculator plus a UMF chart would be easier to look at? 

That eutectic calculator looks very interesting. I can see how it would make some results easier to understand in conjunction with a UMF chart.

Maybe with enough information even a somewhat dim bulb can get the bigger picture.

I always like the idea that a gazillion monkeys could write Shakespeare too.

[thiamant]

What happens with phase diagrams when you combine different fluxes? Like MgO and CaO. 

[Min]

1 hour ago, thiamant said:

What happens with phase diagrams when you combine different fluxes? Like MgO and CaO. 

I would hazard a guess that all glazes have multiple phases, each would be plotted on their own phase diagram with silica and alumina taking 2 positions and the fluxing oxide the 3rd.

Unless you enjoy this kind of thing I don't think it's necessary for 99.999% of people making their own glazes, it's rather a complicated way of looking at glazes to say the least. (edit: that's why I thought the eutectic calculator was so cool)

[thiamant]

So basically the eutectic that melts first goes first, then the second and so on?

[Min]

Glazes are going to melt over a range of temperatures,  particle size is another factor involved in the melt. First part of the melt I would say is when the glaze sinters then the lower melting materials like frits are going to start fusing then when the glaze enters the liquid phase (yeah, bad pun there) eutectics come into play. This is a gross over simplification of what is happening. There is a good article here on glaze melting temperatures and why they are hard to predict. At the end of the day it comes down to knowing your materials and testing.

[liambesaw]

Yeah, so eutectics are like any other flux, in that they melt at a specific temperature.  It's two oxides that have a higher melting temperature than their combination.  It's really wild stuff to get into, and really only important to severe nerds. For the rest of us we have stiff glazes and melty glazes and we let the chemistry work itself out.  Great to understand that say the combination of oxide A and oxide B melt at a lower temperature, but it's all part of a soup that kind of throws everything into chaos.  

I find it interesting, but beyond that, I try to stick with what works. 

[Bill Kielb]

44 minutes ago, liambesaw said:

Great to understand that say the combination of oxide A and oxide B melt at a lower temperature, but it's all part of a soup that kind of throws everything into chaos. 

I think I have been really thankful for the lead tin eutectic point over the years with my soldering.

[liambesaw]

30 minutes ago, Bill Kielb said:

I think I have been really thankful for the lead tin eutectic point over the years with my soldering.

Lol absolutely!  Much better qualities than the silver tin eutectic