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Silica/alumina ratio in glaze = glossy or matt ?

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I use glazes that have been run through Hyperglaze, and there is a lot of info on the recepie sheet that I have yet to know enough to profit from. One piece of info I understand is the the silica/alumina ratio as a guide to how shiney or matt a glazz will be when fired correctly.

 

I have a glaze that is rated 4.6:1 making me think it would be definitely matte. But when fired it is very glossy, even when cooled very slowly. Other glazes in the same kiln at the same firing rate will come out satin or matt, but not this one.

 

Can someone educate me more on this subject? Thanks

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Sorry I missed your question, I've been away for a week. Thanks for responding.

Recipe is;

 

Nepheline sy 47

dolomite 10

ball clay 10

gerstly borate 23

tin oxide 5

zircopax 5 add 3% bentonite

 

 

This glaze is great on darker clays with carvings, but why is it shiney with a S/I ratio of 4.6 to 1?

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^6 , 2195* with a 20 minute soak does perfect tip touching cones all throught my kiln now that the mother board works correctly. Ya!

I do the stepped down cooling that Hessleberth recommends and other glazes are soft satin matt, but not this one.

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Gerstley Borate is low melting at 1600 F, or cone 010. I can't imagine that at 23% it would be easy to make a glaze go matte. Maybe if the rest of the glaze was clay it would be matte?? What you've listed there looks like an opaque glossy glaze to me that would probably start to melt out at about cone 1 - 3.

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I don't know where it came from, but lots of people at the school where I am have used it for years, great on carving, brown if thin, white on porcelaine, always shiney unless very thin, not runny and fired to ^6.

When it is put through Hyper Glaze it is listed as having a silica/alumina ratio of 4.6 to 1.

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4.6 silica to 1 alumina makes sense. That glaze has very little clay, and consists of mostly feldspar, borate and flux.

 

 

 

And my question is, with that ratio, why isn't it a matt glaze ? I was taught that the ratio is a good indicator as to howw matt or shiney a glaze is.

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Here are some articles that can help explain some of your questions. Rather than listen to me it might help more to hear it from the horses mouth.

 

 

This article explains gerstley borate in pretty good detail. This might help you understand the role gerstley borate is playing in your glaze.

http://digitalfire.com/gerstleyborate/material/m-0101v0.htm

 

 

To understand the matte article you should read this article about the glossy base glaze.

http://digitalfire.us/4sight/education/g1214w_cone_6_transparent_base_glaze_121.html

 

 

Then this article might help you understand the matte cone 6 glaze.

http://digitalfire.com/4sight/education/g1214z_cone_6_matte_base_glaze_122.html

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Guest JBaymore

Excellent reference to supply to them there, benhim.

 

Anyone who is not familiar with DigitalFire.com and the Insight glaze chemistry software should do themselves a favor and check it out. A Level II program purchase is a GREAT deal that opens up some awesome online reference materials that are not available for free or in the Level I version of Insight.

 

That Level II is the minimum purchase that I use with my undergraduate students in the ceramic materials course at my college.

 

http://digitalfire.com/index.html

 

best,

 

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

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