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Iron Oxide Used In Clear Earthenware Glazes?


PottaFella

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There seems to be a tradition, at least in the UK, of adding a pale yellow/umber tint to clear earthenware glazes used over slipware etc. I think this is often achieved with addition of a smidgen of iron oxide to the clear glaze recipe? In times gone past I believe these glazes would have also contained lead.

 

What I'm wondering is...

 

What did the lead do to/for the glaze?

 

The iron  oxide colouring now, does that  help the glaze in any way other than aesthetics? Does it make is less prone to crazing for example?

 

Look forward to any replies  :)

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The lead traditionally turns the white slip decoration yellow in earthenware. When I lived in Spain and they joined the EU in 1986 I was under the impression that lead was forbidden. The frits may have been tolerated.

Here you see a traditional English slip decoration using the lead glaze.

post-1954-0-14735600-1502458980_thumb.jpeg

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I have been one of those to publicly decry the use of lead (search for it), but while I was doing that, I was passing recipes to the OP via private messages. I agree completely that there is no substitute (except heavier, more toxic metals). Boron kills iron's beauty. And it is a shame that chapter of ceramics is coming to a close.

 

What I'm not so cool wth is p u b l i c l y posting lead glaze recipes. It's too easy a thing to use unsafely, even the bisilicates. Which aren't so encapsulated as people seem to assume. I do ask you reconsider posting lead recipes publicly, since people do tend to use this forum as a reference guide and ignore petty details like safety and best studio practices.

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You'll find that books and courses tend to form an effort/pay wall the irresponsible don't wish to cross. When I worked as a TA, the apathetic tended not to seek out quality info sources. I can think of a dozen ceramics books in my library with lead glazes in them. But no one checks them out.

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The lead traditionally turns the white slip decoration yellow in earthenware. When I lived in Spain and they joined the EU in 1986 I was under the impression that lead was forbidden.

 

I don't believe that lead in itself turns white slip decoration yellow. Certainly, I have used clear glazes formulated with lead bisilicate over white slips, and no yellowing was evident. I suspect that any historical yellowing was caused by impurities in the lead used at the time (often just dusted on, or crudely washed in place), or deliberately caused by the addition of iron, as in my example.

Lead frits are widely available across Europe, although this position may change, as the law begins to codify information regarding lead as an endocrine disruptor (on top of everything else).

Industry continues to use lead in glazes, of cours

It may be determined more by the clay. I have seen yellow lead/iron glazes as well as green lead/copper and brown lead/managsese  on the low fire clays in Spain. 

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Neil & Marcia:

 

Yellow, buff, light tan'ish colors come from higher titanium levels in conjunction with lower levels of iron in the clay. Same effect you get in certain stoneware bodies. Colorants can be leached from the clay.post-73441-0-85990100-1502492179_thumb.jpg
It is the clay body adding the colorant effects to this glaze, the glaze is crystalline.

Nerd

 

Edit: names inserted, this response was directed to specific comments about clay.

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Yellow, buff, light tan'ish colors come from higher titanium levels in conjunction with lower levels of iron in the clay. Same effect you get in certain stoneware bodies. Colorants can be leached from the clay.attachicon.gifimage.jpg

It is the clay body adding the colorant effects to this glaze, the glaze is crystalline.

Nerd

Tom,

 

In this case the titania is irrelevant. It's just a function of the iron in the red earthenware used in the glaze. You can add 2-4% iron to any clear glaze and will get a similar effect. The depth of colour Sputty's referring to is from 1) the lead and its heavy metal glass optical effects and 2) the different varieties of geologic clay types in an earthenware clay, which can be chlorite, illite, montmorillonite, or kaolin or whatever else--the fact that its impure, messy, plain ol' mud, in other words. Sputty's assessment is spot on.

 

The principle you're applying has its place, but in this case, it's much simpler. If you've got a clear base glaze kicking around, just see what happens as you ratchet up the iron percent by percent. if using a high/mid-fire glaze that ISN'T crystalline and lacks boron, you can take it right up to 20 percent to get tenmoku, kaki, and tessha glazes along the way.

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Was the danger from lead in glazes more to do with the handling of the raw materials – after all it got a very bad press in times gone by?

 

Is lead (esp. if confined to outside of pots or on decorative ware only) more dangerous than other chemicals used in glazes (cobalt?)

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Oly: 2 true stories, although I do not recall what glaze books I read them in.

 

1. The Romans used a raw lead powder to rub the outside of their pottery to produce a glaze. Slaves were used for this procedure, and all died in their mid-20's from poisoning.

2. In more recent times, a potter used lead to make red bowls for her cats to eat and drink from: both cats died within two months.

 

Nerd

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Sputty,

thanks for this. As a classics aficionado, this is interesting stuff. This particular lead glazed ware was relatively rare, with red terra sig comprising the vast vast majority of every day items until after the fall. It was so rare in fact that there are only two (we think) references in the classical literary corpus, "rhosica" being the epithet--from Rhosos in Cilicia; one reference comes from Cicero, who worked in the region for a bit (Cic. Att. 6, 1, 13), the other being Athenaeus in his "Deipnosophistae"--proper citation escapes me.

 

It was my understanding that the glaze came about as a waste product from the metal industry (specifically silver and copper) and represented a re-synchronization of trade knowledge (i.e. working knowledge of glass, metal, and ceramic. From a cursory glance at your article, it seems to support this. What I would love to knoq ia why it didn't happen earlier and came only at the end of the Hellenistic period.

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I do like the idea of a lead-type of crystal clear glaze for my dog jugs. I thought clear glazes would be easy but it has actually thrown up quite a lot of problems.

 

I've also used a premixed powder clear glaze (non-lead) and I'd say that can look a bit plastic-like, especially if it goes on too thick. 

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I do like the idea of a lead-type of crystal clear glaze for my dog jugs. I thought clear glazes would be easy but it has actually thrown up quite a lot of problems.

 

I've also used a premixed powder clear glaze (non-lead) and I'd say that can look a bit plastic-like, especially if it goes on too thick. 

 

You are in search of the Holy Grail!

 

Linda Arbuckle gave the following recipe for a 'pretend' lead glaze - it should have something of the depth of a lead glaze, and similar colour responses:

 

Frit 3110 --------------------- 30.6

Gerstley Borate ------------  33.0

China Clay ------------------- 26.0

Flint ---------------------------   5.0

Wollastonite -----------------   3.0

Strontium Carbonate ------   2.5

Bentonite --------------------   2.0

 

 

Cone 03 - 04

 

You might like to try playing around with it.

Being in the UK, you'll have to substitute something for the Gerstley Borate, and you'll probably want to substitute for the Wollastonite too.

For each 100 grams of Wollastonite substitute 86.1 grams of Whiting and 51.7 grams of Flint, so for the 3.0 in the above recipe, use instead 2.6 Whiting and 1.5 Flint.

You can substitute Calcium Borate Frit, or Colemanite, for the Gerstley Borate, and see how you go.

Strontium Carbonate is available cheaply on that online bookseller named after a long-ish river.

 

I did make a start with developing this, but got side-tracked (quelle surprise...)

 

I have never found commercial glazes fit for (my) purpose.

 

Give it a go!

 

 

 

Thanks very much – I will give it a try, I have something called Gillespie Borate – sold as a Gerstley Borate substitute here in UK.

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