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HarryGateaux

Toasted clay on unglazed side of rim

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Hey

I struggled to find an answer elsewhere so this is my first post! When I glaze the inside of a pot only sometimes the just outer rim goes a toasted/warmer colour than the rest of the outside. What's the reason/science behind that? and is there a way to cause it to happen to the whole outside? 

Thanks a lot! 

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Sounds like the glaze is breaking over the rim?  Meaning that it is very thin there because gravity pulls the glaze down Away from the rim when it's fired.  You can add texture to the piece or apply the glaze very thinly to achieve that look 

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2 hours ago, HarryGateaux said:

the just outer rim goes a toasted/warmer colour than the rest of the outside. What's the reason/science behind that? and is there a way to cause it to happen to the whole outside? 

Most probably the 'toasted' effect is due to solubles migrating from the glaze slurry areas into the nearby regions of unglazed clay body.   
  
I have had reasonable success with spraying raw clay areas with solutions of baking soda, sodium borate (a.k.a. borax), and /or trisodium phosphate (TSP).  The solutions are clear and without suspended solids.  I aim for a concentration of about three quarters of the saturation level of the solubles.    The effect is to marginally change the fired surface with oxides that marginally changes the optical properties of the surfaces.  each "soluble" ingredient will have a similar but different effect.  The phosphates tend to be more reddish than the others on some clay bodies.  

Heavy treatment leads to an unattractive glassy surface.  The effects are different for stoneware and porcelain as well as the amount of grog surface particles - rough vs smooth vs burnished surfaces. 

My technique is to spray a fine mist, allow the area to loose its shine and spray again as apposed to having a strong deluge stream. Application is an art sort of akin to salting soup, adding cream to coffee, or chocolate syrup on ice cream. There will be a period of trial and ERROR as you create a personal application technique. 

As to the science, the soluble reagents will 'tweak' the surface composition to produce a very thin layer that is different from the untreated regions.  The reagents I use readily react with the clay body ingredients to produce very thin transparent glass layer. The optical properties are a function of both composition and layer thickness; surface particle orientation may also contribute to the visual effect. 

LT
 

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As Neil and Magnolia said, I believe it is either volatile or soluble fluxes. Some glazes do this more than others but especially those with higher amounts of Neph Sy, Lithium Carbonate, basically any on the alkaline fluxes/feldspars high in sodium, potassium, and lithium.

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