Matte To Gloss Glaze Chemisty With Al2O3

[neilestrick]

The Si:Al ration doesn't mean squat unless you have a somewhat stable glass. If the fluxes or silica or alumina are way out of limits, then all bets are off as to how the others will affect the look of the glaze. You can increase the silica to a point that it will start to go white and matte, but on paper the Si:Al ratio would lead you to believe that it's the world's glossiest glaze.

 

This has been a great educational thread, and points to what I said a few months back in the Currie thread about limit formulas. If you are doing this for the sake of knowledge, or you don't need durable functional glazes, then a grid like this one is definitely worthwhile. I think we all learned something here. But if you want your grid to have as many useable glazes as possible, then you need to keep all four corners within/near limits. Then you'll see how the Si:Al relationship can affect glossiness. Of course, as John said, there's a lot of other factors, too.

[High Bridge Pottery]

I find it quite useful to test glazes until they break. I guess I am trying to figure out the function of each oxide so then I can make a glaze with any function. Throwing my musings out there for others to comment on. Going to have to buy some alumina so I can really test out changes with it. Really thought I was just increasing the alumina but at least I can see now and hopefully won't make the same mistake again.

[neilestrick]

You can increase the alumina without using alumina hydrate/oxide. You just have to reformulate the recipes that make up the corners of your grid. Basically the clay % will go up and flint % will go down. In fact, it's better to not use it and rely on the alumina in the kaolin and feldspar because it won't melt the same, and in actual use it would be really expensive to use alumina hydrate/oxide when mixing a full bucket. 

[Guest JBaymore]

Building on Neil's comments........

 

Partilcle size is one raw materials SOURCING issue that (as I mentioned above) does impact the way that glaze melts.  Depending on the firing cycle this can change the end result if the glaze is not allowed to fully come to (technical) maturity (which a LOT of our glazes are not allowed to do).

 

The "best" places to start with supplying oxides are in raw materials that already have the components in intimate and often molecular sized contact.   Compared to even the finest grains of 325 mesh silica, the silica molecules that are already associated with alumina and fluxes and so on in a particle of feldspar will be affected by the nearby flux materials WAY before that MASSIVE grain of "pure" silica. 

 

You can think of it as "the small stuff melts first".

 

So you'd want to source the alumina from other materials first.  It is only if you can't get enough alumina into the mixture in that way that you'd result in using something like aluminum oxide or alumina hydrate powder.

 

When a kaolin (any clay, actually ) is heated there is a change that happens called "silica ejection" when it becomes what is known as meta-kaolin.  Those "bits" of silica are literally single molecular size.  They are very reactive with any fluxes present. Getting some of the silica in a glaze in this way is preferable to adding ground quartz rock. 

 

(This is also one of the reasons for dunting in clay bodies that don't have enough glassy phase.... those tiny ejected crystalline silica molecules convert very readily to the cristobalite form.)

 

best,

 

......................john

[High Bridge Pottery]

Thanks for the info Will use what I have in the studio and leave out the Alumina. Plenty of Kaolin. Took a good look at all my feldspars and Neph has the most Al2O3 by far.

 

Had a go at making the 4 glazes, really hard to max out to the top of limit formula the silica or alumina without a glaze being 70% silica. It is a complex dance that probably requires more thought. Making up the flux as I go along, not sure if this ratio was a good choice. Looking at it now I could probably up the silica is Glaze B.

 

[Min]

Were you trying to make a balanced glaze with those new examples? Using Green & Cooper ^9 limits and Insight the 1st one has barely enough alumina and far too little silica, 2nd one looks ok but I agree you can fit more silica in there, 3rd one has only about 1/2 of both alumina and silica and the 4th one has 1/2 the alumina and bottom end of having enough silica. (using ^8 - 10 traditional limits the figures are close to Green & Coopers)

 

I actually took the time to look at the fluxes this time, all within the limits charts I used.

[High Bridge Pottery]

These glazes are the four base glaze that you mix together to make a currie tile. Each corner square contains one of these glazes and then they are mixed across the grid. Somewhere in the middle should be glazes that are within functional limits.

 

The idea is

C - bottom left corner - low Si and Al

D - bottom right corner - high Si and low Al

A - top left corner - low Si and High Al 

B - Top right corner - High Si and High Al

 

Once you mix them in the different ratios is should show me what increasing alumina does up the tile, and what increasing silica does across the tile. That is what I thought I was looking at before and getting confused.

[Min]

But if only 1 is within the limits how can "diluting" it with another which isn't result is a useable glaze? Wouldn't it make more sense to have all 4 within the limits?

[neilestrick]

But if only 1 is within the limits how can "diluting" it with another which isn't result is a useable glaze? Wouldn't it make more sense to have all 4 within the limits?

 

That was my point earlier. If you want to make useable glazes, then yes, there's little sense in having any of those 4 glazes outside limits. However, if you're doing this strictly for the case of knowledge, or if functionality in the 'food safe' sense is not an issue because you're making sculpture, then these types of out-of-limit tests can yield some interesting results and be very educational (as we've seen from this thread).

[High Bridge Pottery]

The one that is within the limits is meant to be over the very top limits but I didn't quite get the chemistry there. That was the best I could do. I am not wanting to make 35 transparent glazes, more see what happens when you are lacking or overloading certain materials, mainly silica and alumina. I find there is interesting stuff out there even if it will never be on a pot intended for food or drink. It's the story the mixtures are telling not what they are actually producing.

[Guest JBaymore]

The most "interesting" glazes are almost always outside limits on something.  The limits one finds are typically defined by "industrial standards". 

 

There is a reason sinks, bathtubs, and toilets look like they do  .

 

best,

 

....................john

[Callie Beller Diesel]

What happens if you take some of the whiting or Dolomite out in order to raise the SiAl to flux ratio, in the name of getting a "broken" glaze for that high Al corner?

[High Bridge Pottery]

Interesting idea, never really thought about me changing the SiAl to flux ratio. It goes from about a 1.5:1 up to 4.5:1.

 

Going to try a few different tests once the bisque has fired and report back.

[High Bridge Pottery]

Ok, so here is a go at making a tile that actually increases in alumina one way and silica the other. Changed the size of my tiles from the currie 5 by 7, felt I could use a 5 by 5. Had to do a little more work on the spreadsheet so I can calculate each glaze but not much.

 

Didn't actually get any squares that were dry overloaded by silica to see if alumina changed that in any way. At least on these tiles it doesn't seem to help the melt like I thought was happening.

 

 

 

Also did one trying to keep the total alumina and silica to flux ratio similar. I actually decided to go from 3.5:1 over to 4.5:1 but it was complicated trying to swap alumina and silica while only exchanging and not adding or removing. Have to say they look quite similar tiles even though they have quite different flux to everything else ratio.

 

 

Both tiles have this flux ratio - cone 9

[Callie Beller Diesel]

I find it interesting that your one tile is pulling apart. It wasn't cracked to start with, was it?

[High Bridge Pottery]

I forgot to say it is also a white stoneware instead of the buff. Had run out of any fresh stuff with that clay.

 

Not sure about the crack, could easily have been from the making or drying uneven. Not taking the most care when making them. This tile was a little thick too so that could have helped drying stress appear. 

[neilestrick]

Lots of unevenness in that tile, too, with all the impressions.