Question About Steven Hill-Esque Approaches To Glazing And Durability

[Kohaku]

As a potter, I've concentrated primarily on Raku... but I do dabble in cone-6 work. Aside from cranking out the odd mug or two, I sometimes make cone six base vessels for my Raku fountains.

 

I'd like to enhance my surfaces... with a particular desire to render things as organic and complex in appearance as possible.

 

When you start thinking about organic, complex surfaces, it's impossible to avoid Steven Hill's influence. I've been devouring his DVD of late...

 

However, I was pretty heavily influenced early on by Hesselberth and Roy's 'Mastering Cone Six Glazes'. The idea that functional ware needed to be tested for leaching, durabilty, fit, etc. kind of burned itself onto my cortex.

 

When I look at the kind of glaze overlay that Steven Hill does... I have to believe that it would be impossible to really evaluate all the combinations for these important characteristics. My impression is that two (or more) glazes can sometimes act in a manner that's not predicatable based on the individual glazes. So... what's the approach? Do you just use a robust base glaze and hope for the best? Do you not worry as long as glazes are in a place that doesn't contact food?

 

Or am I being too paranoid?

[Min]

I am so glad someone else has the same concerns regarding some of his glazes being used on food vessels as I do.

 

Hill seems to have no qualms about using any of his glazes on the insides of food bearing bowls or plates. (granted he does use a liner glaze inside his cylinder shaped pots)  I have his cd, enjoyed watching his techniques but wouldn’t use most of his glazes on pots with food contact.

 

Jen’s Juicy Fruit, Frost Black, SH Copper Ash, Hannah’s Fake Ash (made as per his original recipe not the typo one from the cd) are all short on silica to say nothing about the percentages of copper he uses in some of them. SCM also is short on silica so that is inadequate to draw more silica into the glaze combo mix. Water Colour Green, 7% copper carb on food surfaces?  Nope.  Iron saturates are always short on silica so there is a durability issue there even if iron leaching isn’t an issue.

 

Using a robust base glaze as you suggested would probably help but I don't know how you would measure the interaction between that base and the overlying glazes. Doing a quick vinegar test plus a soda ash one then if they pass those following up with a lab test would cover all the bases.I don’t think that being responsible is being paranoid.

 

Link to a good article, http://digitalfire.com/4sight/education/are_your_glazes_food_safe_or_are_they_leachable_12.html

 

Min

[bciskepottery]

You mentioned the key . . . distinguishing between the surfaces that will be used for food (inside of pitcher, bowl, cup) vs those on the outside and which will not touch food.  As long as your liner glazes are durable and safe, you can work the exterior differently.  If you only use the glaze on the outside, there may be no need to test.  Minimize overlap where practical and may present a potential concern, e.g., lip of a cup.

 

Hesselberth and Roy do a wonderful job on durability and safety; but, as they will admit, the standards they used are basically ones they defined.  If you look at John Britt's book on Cone 10 glazes, his parameters are a bit different -- not much, but still not the same.  So, there is no absolute agreement on what defines durable and what is the appropriate leeching factor, etc.  H&R used water standards; but those are not standards for ceramics and not everyone would agree with using them.  Unfortunately (or fortunately for those who dislike govt rules and regs), there is a paucity of standards. 

[Kohaku]

You mentioned the key . . . distinguishing between the surfaces that will be used for food (inside of pitcher, bowl, cup) vs those on the outside and which will not touch food.  As long as your liner glazes are durable and safe, you can work the exterior differently.  If you only use the glaze on the outside, there may be no need to test.  Minimize overlap where practical and may present a potential concern, e.g., lip of a cup.

 

Hesselberth and Roy do a wonderful job on durability and safety; but, as they will admit, the standards they used are basically ones they defined.  If you look at John Britt's book on Cone 10 glazes, his parameters are a bit different -- not much, but still not the same.  So, there is no absolute agreement on what defines durable and what is the appropriate leeching factor, etc.  H&R used water standards; but those are not standards for ceramics and not everyone would agree with using them.  Unfortunately (or fortunately for those who dislike govt rules and regs), there is a paucity of standards. 

 

Yeah... I'd love to be a little more free-wheeling with exteriors.

 

I'd hate to sell something with overlapping glazes and find out later that the surface had scratched, or that the glaze had shivered because the expansion coefficient was altered. My understanding has been that all bets are off once you start mixing things.

 

Or- to take a specific example... If I used some of Hill's techniques on the interior of a fountain basin, I'd be worried about the colors leaching. My fountains incorporate growing plants in some cases... and various things (from fertilizer to biological run-off) can get into the water as it circulates. I'd like to know that the glaze could handle a certain acidity over time.