Seeking chemically inert glazes
Posted 26 November 2012 - 07:57 PM
But... ceramics have a very earthy (forgive the pun) practical aspect to them as well: they make containers which are rigid, fireproof, and sealable. These are characteristics that meant a lot to early ceramic workers and their cultures. People have been storing household products in glazed ceramics for 3,000 years or more. Obviously vinegar (acid), pickling brine (alkali), whiskey(alcohol), and a myriad of salts and oils have been kept in masonry containers long term. Surely it follows that there were glazes capable of keeping the various solids and liquids from either attacking the container, or being attacked by it. If so, then at some point in history there must have been a lot of (low cone, simple composition, widely used) glaze recipes used for just these purposes.
Where did they go?!?
Like most people I can appreciate a "glassy luster, light crackle, smooth cobalt blue glaze", but if I *use* the vessel, I'll need to know if it's going to deposit toxic cobalt oxides in my raspberry preserves.
So two questions:
1) Do you know of a "solidly food safe" glaze that's got a simple composition and low cone glaze temp?
2) Is there a book or online resource (or should there be one?!?) which covers "practical" glazes?
Thank you kindly,
I've been wondering if a 50/50 kaolin/borax glaze might be possible, but I'd rather have a historic, proven starting point.
Email me or reply to this thread - I'm eagerly listening.
Posted 26 November 2012 - 11:48 PM
Welcome to the forums.
In fact, historically there have been lots of glazes that are NOT up to the standards that we might know about today. Many simple traditional glazes were not all the durable and "food safe". But the fact that the human race is still around speaks a bit to the fact that the whole issue is not "death incarnate". Being concerened is important.... being overly concerned is not.
If you stay with basically non-toxic oxides in the formulation of the glaze, like K20, Na2O,CaO, MgO, Al2O3, SiO2.... then even if the glaze is unstable... it will not leach anything into food stuffs that is an issue. How long the surfaces will remain "pristine" is another story.
"Mastering Cone 6 Glazes" by John Hesselberth and Ron Roy is a great book for understanding ALL cone levels of glazes for durability. (Terrible "limiting" title!)
Go the the Digitalfire.com website and see the info Tony Hansen has on there for a really great resource. And to really understand what you are doing, purchase a copy of Insight glaze chemistry software and take the time to learn to use it well. Tony's websitre will coach you through it.
There are lots of older threads in these forums that deal with this whole issue. Take a look at what might be relevant topic headers of past messages.
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art
Posted 27 November 2012 - 07:24 AM
I would have to contradict the idea of glazed containers being used for storage for 3000 years. Perhaps in some regions , but on the whole it was not common. Egyptians did use a very soft alkaline glaze but it etched easily. Not something good for storing vinegar.The copper turquoise glaze they developed was more common for decorative purposes like the Hippo sculpture in the Met in NY for example. And that technology disappeared for a while . The Chinese did develop high temperatures and better glazes early on ..about 3400 years ago.
In the Mediterranean areas much of the pottery used for storage was unglazed.
And yes, borax and soda were some of the earliest fluxes in making glazes. And don't forget lead, a real early favorite for glaze flux. Lead leaches as we now know and avoid. Soda also needs to be balanced in the recipes to not leach. As John says there are resources out there for developing balanced glazes such as Mastering Cone 6 Glazes by Ron Roy and John Hesselberth
Posted 27 November 2012 - 06:35 PM
I shall certainly purchase the "Cone 6" book you both recommended.
I just want to be sure I expressed myself clearly - as a neophyte that's always a concern - I want to prevent contamination in either direction: from the container to the cargo or vice versa. Lead leaching into my preserves is bad, but so is milk leaching INTO a porous clay container - it'll never be really clean or safe to use again.
Specifically, I'm looking for a simple, low-to-mid cone glaze that will let me store materials and clean the container afterwards.
Posted 27 November 2012 - 10:17 PM
Posted 29 November 2012 - 06:44 PM
I have exchanged email with Tony Hansen of DigitalFire, as well as Matt Richens (referenced on digitalfire.com) and both were friendly and busy at the same time.
Wandering around digitalfire.com, one possible template is G1214W Cone 6 Transparent Base Glaze
It's looking (to me) as if a first cut at the oxides goes something like this:
Along with 2.8 silica and around .35 alumina
Which could be provided with (roughly)
5 parts by mass Sodium Borate Decahydrate (borax,381g/mol) 5kg = 25mol/Na 50mol/B
6 parts by mass Calcium Carbonate (Whiting, chalk,100g/mol) 6kg = 60mol/Ca
4 parts by mass Silicon Dioxide (Silica, 60g/mol) 4kg = 240mol/Si
And enough kaolin to get it all to "stick".
Does that sound "reasonable" to anyone out there?
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