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Iron Oxide Used In Clear Earthenware Glazes?

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oly    13

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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Sputty    73

Whilst you can add a touch of iron oxide to a clear glaze, it's more usual for the glaze to be made partly of red clay. The honey colour achieved is richer, deeper, and warmer - more human. So, for example, my own earthenware lead glaze is simply this:

 

Lead Bisilicate - 75

Red Earthenware Clay (dry weight) - 25

 

That's it. So I would simply use the same red clay as the body, but dried. I used a white earthenware clay in a similar fashion to get a non-tinted clear glaze.

 

The lead is the flux for the glaze. (Some say that it also gives a very slight yellow-ish cast to the glaze, but that's not something I've noticed, at least not with the lead frits. It may be that raw lead does that).

 

Lead glazes are amongst the most beautiful glazes you will find anywhere - their depth, sparkle, colour response, and sheer presence is quite, quite stunning. Think lead crystal glass, and then add earth, warmth and humanity.

 

But of course lead is somewhat poisonous, and people will counsel you most strongly to avoid it, even in frit form. The problem is that there really is no substitute - there's nothing that even begins to mimic a lead glaze. Sure, you can get a clear glaze made with other stuff - but it's not the same. It's exactly that problem that led me to (mostly) give up earthenware. Anything other than a lead glaze seemed a distinct second best. Even the best majolica glazes are lead-fluxed - Alan Caiger-Smith used an utterly beautiful tin opacified lead glaze, which I defy anyone to get remotely close to without using lead (or tin!).

 

There are still plenty of European potters using lead, especially here in France, where the country pottery tradition produced some of the finest 'peasant' pottery to be found anywhere. Equally in the UK - there are still quite a few who use lead (in frit form), and produce superb ware.

 

Anyway, I have enthused at length about lead in other threads on this forum (do a search), and equally others have gone apoplectic over even the very mention of the stuff. It is rather dangerous, and I will be told off...

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Marcia Selsor    1,301

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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Tyler Miller    331

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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Sputty    73

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 course.

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Sputty    73

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.

 

But glaze recipes using lead are widely available in many, many contemporary books dealing with earthenware. Off the top of my head, I can think of at least a dozen books I own which have page after page of recipes in them containing lead frits. These are recently published books, still in print. They are in libraries, in bookshops (including that online thing) - there's no secret knowledge here that I'm somehow revealing.

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Tyler Miller    331

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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Marcia Selsor    1,301

 

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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Sputty    73

 

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. 

 

 

That makes sense.

 

Incidentally, I found the following at a Spanish pottery supplies store:

 

Minio de Plomo

 

Red Lead, or Lead Tetroxide - forget frits - why not just go for raw lead, and do it properly?

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neilestrick    1,381

It was always my understanding that the yellow color came from the clay, not the glaze.

 

As for the use of lead, unless you're ready to take on the health risk and the liability of having your work out there in the hands of the public, then stay away.

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glazenerd    816

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.

Edited by glazenerd

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Tyler Miller    331

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.

Edited by Tyler Miller

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bciskepottery    925

What I'm wondering is...

 

What did the lead do to/for the glaze?

 

 

 

Lead has a low melting point, so it was easily used in low fire/earthenware glazes. 

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Tyler Miller    331

Tyler:

See the edits in my post above: I was specifically talking about clay, not glaze chemistry.

My answer stands. Buff, yellow, gold, etc. comes from iron, not titania (edit: in the case of clay bodies as well). Edited by Tyler Miller

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oly    13

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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glazenerd    816

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    73

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?)

 

Certainly those who use lead today in industrial or semi-industrial contexts are obliged to have regular (monthly) blood tests to make sure that their exposure is not at dangerous levels. The problem with lead is that it remains in the body, and accumulates.

Lead glazes themselves are problematic if they leach lead under acid attack - from foodstuffs, for example. It is perfectly possible to develop a lead glaze which is food-safe - after all, industry does exactly that. Testing of the glaze is clearly advisable (and easily done at laboratories - you have several options in the UK). There can be an exacerbation of the dangers if the lead glaze is over certain other elements in slips - copper in particular.

If the ware is decorative, I would say that the danger to the consumer is negligible. My own stock of lead bisilicate is being used up on the outside of garden pots.

 

Just to make matters more interesting, there is a further argument against the use of lead, which another forum member (forget who - apologies) brought up in a different thread. The environmental impact of mining the stuff is not insignificant - the suggestion was that leaving it in the ground might make the most sense.

 

If you are interested in trying to duplicate the aesthetic qualities of a lead glaze, but without the lead, then I believe that the inclusion of strontium carbonate (sounds lethal - it isn't) in a glaze is supposed to give something of the colour response and limpid depth of lead. I have had little success, but occasionally try something along those lines when I'm feeling optimistic.

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Sputty    73

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.

 

Nerd

 

Here you go - enough glaze and clay chemistry to keep you happy for a while:

 

Production technology of Roman lead-glazed pottery and its continuance into late Antiquity

 

It's got graphs, tables, and everything! Also available as a PDF.

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Tyler Miller    331

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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oly    13

The lead I'm most worried about is the stuff in my teeth :(

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Sputty    73

The lead I'm most worried about is the stuff in my teeth :(

That's not lead - at worst, it's a mercury amalgam, depending on the age of the fillings. That in itself might give you pause for concern, but in general such fillings are still held to be safe, and are still widely used.

Depending on the circumstances, I believe that other formulations are available, including various resins.

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oly    13

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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Sputty    73

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!

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oly    13

 

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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