Silica and gloss

[Crooked Lawyer Potter]

Generally speaking, if one wants to increase or decrease the "gloss" of a glaze, is it as simple as adjusting the amount of silica up or down?  Does varying the silica content require some other adjustments or have some other effect on the glaze?

[dhPotter]

If you are using glaze calc software you will need to raise the silica and lower the alumina. Pay attention to the Si:Al ratio with the Si being much more than the Al. Others here can tell you what ingredients will raise or lower the 2 items. Think about replacing some ingredients, such as replace Calcium Carb with Wollastonite. I am not chemically competent to tell you much about other ways to raise and lower the ratio.

[Callie Beller Diesel]

There’s a few different mechanisms that create matte ness in a glaze, so it depends a bit on which one(s) are at work in the recipe you’re thinking of. But changing the amount of silica is one way that it *can* happen, yes. 

Changing the proportion of any one ingredient will affect the glaze to some extent or other, because all the parts work together to get that given result. Some ingredients will need to be changed more than others to make a noticeable effect. For instance, you’d notice a difference of 0.1% of Cobalt, but not of EPK.

Assuming you’re talking about a balanced glossy glaze suitable for food use, there are parameters of flux, alumina and silica  you want to stay within to keep that balance. If you have a glaze that starts off well within appropriate ratios*, you could change the silica only, and just have the glaze get glossier without changing the other characteristics too much. At least until the silica becomes oversupplied, and it starts being underfired, anyways. If the flux ratios, or the alumina are towards the edge of that balanced range (too much or too little of either) and you start playing around with the third corner of that triangle, you might have to compensate somehow, but it depends what characteristics you’re trying to change and which ones you want to preserve.

There’s a lot of if-this-then-that to this scenario. Have you got something specific you’re trying to adjust, or are you just asking for general information?

 

*if a glaze is unbalanced, it doesn’t mean it’s a bad one to use at all, it just means it’s not ideal for food use. Glazes are used on more than utilitarian pottery.

[Crooked Lawyer Potter]

 

1 hour ago, Callie Beller Diesel said:

There’s a few different mechanisms that create matte ness in a glaze, so it depends a bit on which one(s) are at work in the recipe you’re thinking of. But changing the amount of silica is one way that it *can* happen, yes. 

Changing the proportion of any one ingredient will affect the glaze to some extent or other, because all the parts work together to get that given result. Some ingredients will need to be changed more than others to make a noticeable effect. For instance, you’d notice a difference of 0.1% of Cobalt, but not of EPK.

Assuming you’re talking about a balanced glossy glaze suitable for food use, there are parameters of flux, alumina and silica  you want to stay within to keep that balance. If you have a glaze that starts off well within appropriate ratios*, you could the silica only, and just have the glaze get glossier without changing the other characteristics too much. At least until the silica becomes oversupplied, and it starts being underfired, anyways. If the flux ratios, or the alumina are towards the edge of that balanced range (too much or too little of either) and you start playing around with the third corner of that triangle, you might have to compensate somehow, but it depends what characteristics you’re trying to change and which ones you want to preserve.

There’s a lot of if-this-then-that to this scenario. Have you got something specific you’re trying to adjust, or are you just asking for general information?

*if a glaze is unbalanced, it doesn’t mean it’s a bad one to use at all, it just means it’s not ideal for food use. Glazes are used on more than utilitarian pottery.

Thanks for taking the time to reply in depth. 

Really just for general info.  I'm a hobbyist with no formal training and I struggle to increase my very limited understanding of glaze chemistry.   Another part of my learning curve is an attempt to fully understand the obvious power and convenience of glazy.org  

I suppose what started my thinking about this was one of my standard glazes:  Stoney Matte

Stony Matte  

Neph sye.    42.5

Kaolin.  24.5

Whiting.    17.5

Talc.   14.2

Silica.   1.3

 

I love this glaze when it works -- but I have problems with it "disappearing" sometimes.  Instead of a nice smooth stoney finish it seems as if the glaze just burned off the piece leaving what feels like bare, rough clay with only traces of glaze evident on it.  My thought was to increase the very nominal amount of silica in it to produce a more robust coating of glaze.

 

[Min]

I believe you fire to cone 6 @Crooked Lawyer Potter? Please correct me if I'm wrong on this.

I've commented before on why I don't care for the Stoney Matte recipe you posted above for cone 6 functional pots.

Given your comments about having limited understanding of glaze chemistry I won't enter this into a glaze calc program (like Glazy) but will try and explain why I don't expect this recipe to make a durable cone 6 glaze for functional pots by going over the percentages and materials in the recipe. 

So, having a look at just what the recipe is and taking it apart from there...

Stony Matte  (^6?)

Neph sye.    42.5

Kaolin.  24.5

Whiting.    17.5

Talc.   14.2

Silica.   1.3

First ingredient is just over 40% Nepheline Syenite. Couple potential problems with having so much Nepheline Syenite in a glaze, first off it's going to supply a huge amount of sodium to the glaze. Problem with this is if the glaze is fully melted (I don't think it will be at cone 6 even if it appears melted) it will more than likely craze. If it isn't crazing then that supports the argument that it isn't fully melted. When a glaze isn't melted properly then it will not be durable, this means things like leaching and cutlery marking. Second problem with having this much Nepheline Syenite is if you do get the glaze to fully melt (by adding zinc or boron, more in a minute) then it's more than likely to craze badly.

Kaolin, whiting and talc amounts are okay.  Given the amount of Nepheline Syenite plus having nearly 25 kaolin this glaze isn't going to melt without zinc or boron. Tiny bit of silica added but silica is also being supplied by the Nepheline Syenite and the kaolin. 

What is missing from this glaze is something to get a good melt at cone 6, that being either zinc or boron. It's my guess that this recipe was originally a cone 10 glaze that someone started using at cone 6 and it just isn't melting. Is this glaze white? If it is then there is another indicator it's very unmelted as there are no opacifiers in the recipe.

So, back to your question about adding silica to gloss up the glaze. Yes, this will work if the glaze has enough fluxes or boron to melt the additional silica. (Boron acts as both a glass former and a flux.) Problem is I don't think your glaze is melting well already, add some silica and it won't necessarily be able to be taken into the melt, which will just be exacerbating the issue.  Re your comment about the glaze looking like it's burned off, my hunch would be that in those areas the glaze is even more under-melted, it won't have burned off. I wouldn't try and fix this glaze with silica and or adding boron. I'ld look for a new recipe that has the qualities you are looking for and start testing that. 

[Hulk]

oh yeah, Stoney Matt
Problem with Stoney Matte Glaze "Disappearing" - Clay and Glaze Chemistry - Ceramic Arts Daily Community

added: the recipe, glaze name - spelled with "-ey" - and the OPs description in this thread all seemed familiar. 

I'd still want to know how the thickness of the glaze coating is controlled* - is it thinner where the problem manifests?
What clay(s), firing range. Is the glaze crazing? Are you lining functional ware with this glaze?
Looking in the other direction, what clay, firing range, what look, feel and other characteristics are desired?

The one glaze I use that isn't gloss, more like satin, has high silica, alumina, and boron (as expected in a cone 5/6 glaze), melts really well, doesn't move much, and goes on well.
 

*The application characteristics would be the other topic, my guess - i) glaze and ii) application.

[Min]

Thanks for posting the link Tom. I hadn’t found it under the op’s other name he posted with.
From an image on that link looks like the disappearing areas are thinner glaze as the glaze drips are white like the other pots.

 

[Callie Beller Diesel]

+1 to everything Min said. 

Glaze calculation software is really great for figuring out exactly what proportions of the oxides in a glaze are. Because we work with minerals made of multiple oxides, more than one material may supply the same oxide. While you usually think of a feldspar as a flux because it’ll have the primary source of sodium, it’s tempting to think of it as an ingredient that will make a glaze runny. It can. But it is also a source of alumina, which will make a glaze more viscous. But what happens depends on what else is in there too.

Glaze software takes the chemical analysis of all the materials in a glaze, and calculates how much of each oxide is present in that specific recipe. It adds up all the silica, alumina and different flux metals and presents it as both actual numbers and as proportions relative to each other (UMF numbers, flux ratios and silica:alumina ratio, etc.). Because we also know that if a glaze stays within certain proportions it’ll do certain things (eg. stay matte or go glossy), you can compare your UMF table to one of a few sets of limit formulas, or you can see where the glaze is on a Stull chart that’s overimposed on a UMF chart, like in Glazy. Limit formulas by themselves are by necessity a bit vague, but they will tell you if an ingredient is really out of whack. Stull charts can tell you if a glaze is over or under fired, or if it’s supposed to be matte/glossy given it’s chemistry. There are caveats, and I recommend reading through Glazy’s Help section, starting here if you already understand what molar weight is. If you don’t remember molar weight from high school chem, just scroll up the page a bit.

To describe what Min said another way:

When I enter this Stoney matte glaze into the glazy calculator, long story short, it’s in the matte zone of the Stull chart by the skin of its teeth (the medium green triangle on the left). It’s also in the crazed zone (grey overlap part).  It’s not super balanced,  which means it’s probably a bit persnickety even at the cone 10 or 11 Stull was originally designed to describe. You could probably fix this one if you were firing to cone 10. Adding silica would definitely make it glossier, and probably help with crazing. You’d have to make a real world test to check for cutlery marking or actual glaze fit though. 

But because the actual alumina amount is so high and the fluxes are all more active at higher temperatures, it’s just plain underfired at cone 6. When I try to add Gerstley borate, the silica:alumina ratio stays close-ish to what it was, but it drops the glaze farther down into the matte+crazed zone. Because GB supplies more than boron, I tried with just Zinc Oxide, but it did the same thing only worse.

If you’re searching the Glazy database for a better replacement, look for recipes that show up on the chart as a dot with some orange on it. The orange indicates the presence of boron, which means a glaze that melts earlier.

[Crooked Lawyer Potter]

Thank you Callie.  Very helpful!