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lauren179

Which Silica To Use?

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Hello,


 


I'm making and testing glazes in my hunt for a satin matt glaze.  I came across this recipe by Tony Hansen, which sounds almost exactly what I'm after, but I'm not sure which silica to use.  Apologies if its a basic question but I'm still relatively new to this!


 


The recipe is here - https://digitalfire.com/4sight/recipes/cone_10_silky_dolomite_matte_base_glaze_39.html


 


Then I am hoping to add oxides to make a black glaze, would anyone have any ideas of which oxides and how much?  


 


Many thanks!!  


 


Lauren


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Guest JBaymore

The "standard" for silica used in glazes is 325 mesh.  Depending on your supplier........ it will be listed as flint or quartz or silica in their catalog.  The common silica at least around here in the north east is a brand name called Silcosil.

 

Another commonly available silica is also 200 mesh.  That tends to get used in clay bodies, although a material's particle size DOES affect how glazes look in different firing cycles.  Unlike glass batches, our firing cycles sometimes do not let a glaze "fine out"... meaning have all reaction come completely to the end result predicted by molecular glaze calculation.  In these cases.... big particles sometimes can REMAIN big particles ....and affect the look (and stability) of a glaze.

 

Note that changing this G2571A glaze to a black can also change the fired characteristics that Tony is listing there.  The "safest" way from a leaching standpoint is to use a black commercial glaze stain.  Lacking that approach, overload the glaze with metallic coloring oxides like iron and cobalt (and maybe manganese depending on the intended use).  Using a lot of coloring oxides can sometimes also change the surface quality... from a matt-ish to more glossy in reduction environments.

 

 Hope this helps.

 

best,

 

....................john
 

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Keep in mind 2 other things: 1) most of the recipes on digitalfire are intended as good starting points for testing: Tony weeds out the ones with bad chemistry, but there is still work to be done. This iteration will in all likelihood be pretty close to what you're looking for, but it won't be perfect. You will have to tweak it somehow. 2) application with most of Tony's glazes are critical to their success. He goes on about glaze density and flocculation for a reason. Doing a triple dip test should be considered essential.

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Would there be any reason not to use 325 mesh in a clay body or slip?

Yes. Cristobalite formation. Silica is very refractory, but the finer it is the better it melts. Too much melted silica in a clay body can lead to cristobalite forming, which causes dunting.

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I just looked at data sheets for sil-co-sil 52, their 325 mesh product, and the sil-co-sil 75, what I see sold as their 200 mesh product by various ceramic suppliers. 

 

Depending on which plant it comes from there are differences in what you get, but all in all, most of the particles are 325M or smaller.  What I'm seeing in these data sheets is that only 8.8% difference in the amount passing through a 325M screen.  there is very little 200M particles in this stuff, 2.2% is the highest amount in these data sheets.  Does that small amount of larger particle sized material have that serious of an effect on the melt of a clay body, and the amount of cristobalite formed?

http://www.ussilica.com/sites/ussilica.com/uploads/files/product-data-sheets/industry/glass/SILCOSIL75-Columbia.pdf
http://www.ussilica.com/sites/ussilica.com/uploads/files/product-data-sheets/industry/building-products/SILCOSIL75-Pacific.pdf

http://www.ussilica.com/sites/ussilica.com/uploads/files/product-data-sheets/industry/building-products/SILCOSIL75-MillCreek.pdf
http://www.ussilica.com/sites/ussilica.com/uploads/files/product-data-sheets/industry/glass/SILCOSIL75-Berkeley.pdf

 

http://www.ussilica.com/sites/ussilica.com/uploads/files/product-data-sheets/industry/building-products/SILCOSIL52-Columbia.pdf

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Easy answer is no. A couple percent silica which does not pass 200 mesh will not matter. In fact, it is the other end of the particle size distribution you should be worrying about.

 

The cristobalite comes from an excess of finely-sized free silica in the clay body materials. "Free" in this case means surplus to requirements for the clay body to form glassy phase, mullite, etc.. ie, all the reactions needed for a clay body to properly vitrify and mature, which involve very specific amounts of silica. But what these amounts are is very difficult to determine because they depend on so many variables, such as clay body recipe, firing schedule, top temp, etc. And then there are user-determined issues like workability of the clay body, plasticity, etc.. which also come in to play when deciding on what kind and how much silica to add.

 

If there is "left-over" fine silica after all these reactions have taken place, it can quite easily turn in to cristobalite during the firing, with unhappy consequences for the pot, namely dunting. Large particle (low surface area) silica generally doesn't contribute to cristobalite because it simply doesn't melt nearly as well as small particle (high surface area) silica.

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