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Achaemenid Glaze And "the Origin Of Glazes" Essay

alkali glaze Persian empire glazed bricks

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#1 Tyler Miller

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Posted 09 August 2014 - 01:53 AM

I just shut off the kiln for the night and I thought I'd share some of the reading I've been doing while the firing's been going on.  A while ago I posted a little something about Achaemenid glazes.  I've kept thinking about them since then, curious about figuring out a formulate for myself.  I found a very interesting essay here (http://www.gustav-we...s-of-Glazes.pdf) on the origin of glazes.  I highly recommend giving the essay a read, it's very insightful about just how glazes came about.

 

"Chemical analyses (Hedges 1982) from the Achaemenid, Parthian and Sassanid periods (from 700 B.C. revealed glazes with the following boundary val- ues: 6-8 % Na2O, 2.5-4 % K2O, 4-8 % CaO, 2-4 % MgO, 2-3 Fe2O3, 4-8 % Al2O3 und 65-75 % SiO2. After a phase diagram by Morey (1930), this ratio of alkali to earth alkalis and silicic acid needs a firing range of 900°-1100°C. These glazes were ap- plied to a body with 16-17 % CaO, 5-6 % MgO und 50 % SiO2, which corresponds to the lime-rich clays of the Middle East."

 

 

 

10% soda ash, 10% potash feldspar, 10% dolomite, 10% kaolin, and 60% silica puts you right in the ball park of that chemistry.  Historically, this would likely have been soda/natron and local sand, just like Egyptian glass. 

 

Edit:  It occurred to me that I should share how I arrived at that recipe.  The essay talks about plant ash and sand as the origin of this type of glaze, but I'm not sure how realistic this is when trying to replicate the result.  I tried every known plant ash, and I couldn't make it work.  There are plants in the mediterranean and elsewhere that take up soda and potash in tremendous quantities, e.g saltwort (salsola soda), but I don't have those, and it was later found in ancient glass making that substances like natron form a better glass than ash.  In fact, it wasn't until the breakdown of trade routes that ash became important in glassmaking again.  

 

 

Thought I'd share :)



#2 Babs

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Posted 09 August 2014 - 02:09 AM

How does that compare to Eygyptian paste?



#3 Marcia Selsor

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Posted 09 August 2014 - 06:59 AM

I like the explanation of the environment and the soil's development of chemical content. Very interesting. It does sound like Egyptian paste with the migration of the body chemicals helping to flux the surface.

 

Marcia



#4 Tyler Miller

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Posted 09 August 2014 - 11:31 AM

Babs,

 

I think Egyptian paste and the glaze I posted are very similar indeed.  Modern Egyptian paste recipes I've seen are nearly identical in composition, but they seem to have more feldspar, perhaps used as a body flux.



#5 Bob Coyle

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Posted 12 August 2014 - 11:04 AM

WOW! Thanks for the reference Tyler. I got interested and went to Weiss's main page http://www.gustav-weiss.de  This guy is amazing! I also love the great display of his glaze painting and sculptor.

 

Spent about three hours reading his essays and looking at his art. This guy is a true ceramic philosopher not to mention a fine ceramic artist.






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