Silica and Alumina Limits

[HenryBurlingame]

Hi all, I've been brushing up on my glaze chemistry since it has been a while (I don't remember the stull chart being so popular when last I made glazes) so I decided to take 'The middle glazes' workshop from the ceramics materials workshop website.  It has been pretty good, but one thing that has been at the back of my mind is that Matt never discusses silica and alumina levels in regards to durability of a glaze. In fact, there is one point where he explicitly states that glazes low in silica and alumina are just fine, the only issue being that it is difficult to make glazes very low in silica and alumina (while still having a suitable flux ratio) with materials that are commonly available in the studio.

So I went through my old glaze library to my Mastering Cone 6 Glazes book and their first two rules for a durable glaze are: 1. Have enough silica, and 2. Have enough alumina.

Has anything changed with regards to minimum silica and alumina levels since I was using limit charts to do everything ? It looks like Hesselberth and Roy say 2.5-4.0 for silica and 0.25-0.5 for alumina for a durable glaze, while ceramics materials workshop says any levels are ok as long as the glaze fully vitrifies and the flux ratio isn't too far from the 0.3:0.7-0.2:0.8 range.

 

Edited August 17 by HenryBurlingame

[davidh4976]

Matt says that the limits are meaningless.  Every other expert I have read says that limits are useful tools, but not perfect.

In doing conversions of our studio glazes to avoid using Gerstley Borate, Custer Feldspar and other ingredients that are no longer available, we have been satisfied with results using the following limits. At least for glazes that are meant to be durable.  The limits are not so useful if you are trying to get reactive glazes with emphasis on looks instead of durability/leaching. This is what worked for me, but certainly doesn't guarantee anything.  I was also shooting for R2O:RO around 0.3:0.7 but was OK in the range of 0.28:0.72-to-0.2:0.8

Cone 6: Al2O3 = 0.32-to-0.45; SiO2 = 2.6-to-4.0 with 3.5 seeming optimal

Cone 10: Al2O3 = 0.32-to-0.45; SiO2 = 3-to-5

 

[Bill Kielb]

1 hour ago, HenryBurlingame said:

Has anything changed with regards to minimum silica and alumina levels since I was using limit charts to do everything ? It looks like Hesselberth and Roy say 2.5-4.0 for silica and 0.25-0.5 for alumina for a durable glaze, while ceramics materials workshop says any levels are ok as long as the glaze fully vitrifies and the flux ratio isn't too far from the 0.3:0.7-0.2:0.8 range.

Just my take
I am not aware of any changes nor real purposeful tests Re: durability and incremental combinations of silica and alumina. To that, limits differ somewhat between sources and also have fairly large ranges. Katz I think takes the approach that you can chase the silica / alumina up the Stull chart by heat map so to speak, so you can always melt it. Additionally by materials, it’s hard to decrease levels - neph sy being a typical minimal source component. So being able to make things with truly low levels are limited.

Limits to me are guidance, but not so specific as to be absolute. A good example would be any true matte that I make that is stiff. Turns out boron is helping things melt as planned and has  tested well in durability for me. I think the old adage that more silica and alumina is generally a good thing for durability is guidance and a bit relative with respect to what is more? There are tested cone 04 glazes more durable in some respects than cone 6 and cone 10, with indications that boron is central to the durability. I do believe Katz was able to show some correlation with flux ratio and durability but again, like limits, it seems more of a guidance or general affirmation that something is likely durable or my preferred perspective moving significantly away for 0.3:0.7 could indicate non durable. Just as moving too far from whatever limits you are using could have the same meaning.

Relative descriptions such as get enough Silica and Alumina are to me just that, relative so I think a general guide. Testing seems to be in order regardless. So back to mastering cone 6 - have enough Silica and Alumina? What is that exactly? And of course more is likely better.

Just my take though.

Edited August 18 by Bill Kielb

[Min]

Simple experiment to show that silica and alumina levels are important to glaze durability. Take a basic ^6 glaze in which the silica and alumina are easy to remove.  Marc's ^6Clear , https://glazy.org/materials/55849, would be a good candidate to test. Add 1.5 copper carb to the original glaze (in which both silica and alumina are within decent ranges) and 1.5 copper carb to the same glaze without the 15 EPK and 26 Silica (and re-totaled to 100). The altered glaze will likely run so a catcher dish would be required for the test sample. Pinholes expected with the copper addition but not an issue for this test.

Fire both test samples then run a simple lemon / vinegar test and cutlery marking test.

Could take this one step further and swap out the nepheline syenite for a pot spar, re balance the formula  so the total of the sodium + potassium are equal and see if it's more scratch resistant using the pot spar. (probably will be)

8 hours ago, HenryBurlingame said:

1. Have enough silica,...

When I started learning about glaze chem the go to for adding silica to a glaze was to use the less expensive 200 mesh silica, since then it's been shown that a finer 325 mesh silica can dissolve better in a glaze. Doesn't seem to matter to some glazes but this difference alone can be enough to stop crazing in others. Finer the mesh, the easier it is to dissolve.

Edited August 18 by Min
clarity

[High Bridge Pottery]

I always find it interesting to try and figure out where these numbers come from. If you start with Hermann Segers work from 1886 titled "Pyrometers and the Measurement of High Temperatures. Standard Cones" he first figures out that a silica alumina ratio of 8 silica to 1 alumina makes the "most fusible" glaze and goes on to determine that a ratio of 0.3K2O to 0.7 CaO is the most fusible flux ratio.

 

 

 

 

I think from this you can pretty much see where the glaze limits started from with an aim to defining the "most fusible glaze".

There has been a slight misinterpretation that most fusible also means the most durable. If we look at a table from Edward Orton Jr paper "On the production of an easily fusible glass without the use of lead or boracic-acid" (there's a typo in glaze 2-D and it should be 0.25Na2O and 0.75 CaO) it looks to me like anywhere up to 0.6Na2O is perfectly acceptable for making a durable glass and while 0.3:0.7 gives you the lowest dissolved glass there's not much difference all the way up to 0.6:0.4.

 

The reason he's down at 2 silica and no alumina is because he's starting from a soda lime glass.

 

 

 

 

[HenryBurlingame]

Thanks so much all, this is very helpful info!

On 8/17/2024 at 7:54 PM, Min said:

since then it's been shown that a finer 325 mesh silica can dissolve better in a glaze.

Thanks Min! Hesslberth and Roy test alumina and silica limits by measuring copper leaching and it does go up with both lower silica and lower alumina. Looks like if their limits are followed you wouldn’t be able to even get a true matte glaze though... surprised Katz never says anything about this, a lot of his glaze recipes are waaaay down close to the bottom right corner of the stull chart.

Also, are there any issues if you go even finer mesh than 325? Like 400 mesh (which my closest supplier is now carrying instead of 325)?

Edited August 23 by HenryBurlingame

[Min]

29 minutes ago, HenryBurlingame said:

Also, are there any issues if you go even finer mesh than 325? Like 400 mesh (which my closest supplier is now carrying instead of 325)?

That would be fine. (no pun intended)

[HenryBurlingame]

Another interesting tidbit I noticed listening to the ceramics materials workshop. Katz talks a lot about "true matts" being glazes in the matt region of the stull chart, so he only considers alumina matt glazes "true". In the mastering cone 6 book Hesselberth and Roy specifically say that they could not formulate an alumina matt glaze that satisfied their durability goals (leaching levels were at least double their target levels). Their theory was that the aluminum-containing crystals that form in the glaze are forming as alumino-silicates thereby pulling silica out of the main melt and making the formation of a glaze as stable as they wanted impossible. They did not have issues with making durable high calcia, high magnesia, or high strontia matts.

Edited September 7 by HenryBurlingame

[High Bridge Pottery]

It's an interesting theory although I would think that in the high Ca, Mg and Sr glazes they will also be pulling silica out the melt when making crystals but maybe the extra charge on the Al is taking out more silica.

 

The matt debate has been going on for a long time. You might be interested in this thread but I wouldn't put too much thought into them talking about oxygen ratio as I have come to the conclusion that it's not much use.