Multiple Alkaline Earths

[HenryBurlingame]

I am wondering how important it is that a glaze contains multiple alkaline earth fluxes. I have seen it recommended in several places that it is best to use a combination of alkaline earth fluxes, and I noticed, for example in my mastering cone 6 glazes book, that hesseberth and roy will throw a couple % of talc into a recipe for no obvious reason other than to follow this guideline and add a bit of magnesium into an otherwise pure calcium-alkaline-earth glaze.  Am I fine using just calcium for the alkaline earth flux if I don't need any of the other alkaline earths for any specific reason (texture, color, etc.)? I even saw a glaze with only strontium as the alkaline earth and it seemed to work just fine.

Edited September 9 by HenryBurlingame

[C.Banks]

I suspect this is a sticky question.

Rhodes recommends calcium between 0.5 and 0.7 iirc but whether this applies to earths as a whole or cone 6 glazes is beyond my limited understanding.

Magnesium helps glaze fit iirc?

I know high-fire glazes with ratios high in alkaline earths over akali metals can work just fine for years but can only offer to bump this topic out of curiosity.

*

"MgO is very valuable for its lowering effect on glaze thermal expansion (this is one reason why MgO mattes can be made very resistant to crazing)"

https://digitalfire.com/oxide/mgo

if I was more ambitious I'd test more to see how little/much MgO will help prevent crazing but my list is already long enough. I'm tempted now though to see how a couple ~ 0.1 : 0.9 glazes really do stand up. Maybe splitting up the RO's will help.

I've added whiting in small amounts to bring amounts over 0.5 and always thought it looked kinda odd but never tested it to see how much differenc it made

interesting question that gets a person thinking

Edited September 9 by C.Banks

[Hulk]

I don't have craze reduction/elimination physical test history where MgO was the only change; in each case there was sodium and potassium change (less), and then introduction of lithium (via Petalite). The last adjustment in my liner glaze Odessey*, more lithium, boron, magnesium, Zircopax; tiny bits less of several other oxides.
Lithium, powerful stuff.

The software's calculated expansion coefficient, that's something! I'd done many many "trials" with the software, tweaking the ratios to see how the COE changes.
...and I do believe calculated COE can be helpful, especially when the change(s) are narrow/focused.
I also believe the calculated COE can be misleading - where the results don't quite match up, especially when comparing very different recipes.
 

*"last adjustment" - that was before the cost of Petalite went up and up! I have on my short list staring on a new liner glaze without Petalite...
 

 

 

Edited September 9 by Hulk
spellin'

[Callie Beller Diesel]

I don’t have a direct comparison of high calcium vs calcium and magnesium glazes, but I did do a comparison of a couple of high calcium mattes in the past year. They both had flux ratios of 0.2:0.8, but one had a lower silica:alumina ratio. (5.1 vs 4.9) They also have different proportions of the same materials, although the higher alumina glaze also has a bit of titanium, which will affect things. (The adjusted Si:Al2O3+Ti02+NiO is 4.0, which is significant.) Both glazes are well melted at cone 6, as deliberate drip marks smoothed out on both glazes.

For reference, here’s Sue’s Calcium Matte (less silica) and Eggshell (more silica) on my Glazy.

Sue’s Calcium Matte (SCM)  crazed on most clay bodies I tested, although not the one I use most with a minor adjustment. It was also subject to some acid etching, depending on the colourant used. SCM can cutlery mark at cone 6, but less so when melted better either with more heat (cone 7) or more silica added.  Eggshell doesn’t scratch, but does cutlery mark. It showed no sign of acid etching on any colourant. I’m not that upset by cutlery marking, as it means your glaze is durable, and you can take it off with a magic eraser.

The colour response on both glazes is quite pretty, and would be ruined by the addition of magnesium, but the quality of the colour is different. Eggshell is very pastel and very opaque, and SCM is quite vibrant.  Both will give you 2 different but pretty turquoise colours from copper, but if you add silica to adjust the crazing or gloss level on SCM, you get a more true green with no blue. (That test I don’t have pictures of, sorry.)  Interestingly, Eggshell turns an interesting mottled pink/purple when you add 2% black mason stain (6600). First time I’d seen that stain break like that IRL, and it reminds me of some of the old reduction barium mattes.

I have not removed the titanium from the Eggshell base to see how it affects either crazing or durability.