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Topic: Replacing white lead in gesso sottile  (Read 13843 times)

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Offline Katzenjammer

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Replacing white lead in gesso sottile
« on: May 09, 2008, 07:25:58 PM »
The traditional receipe for the gesso used in raised gilding for illuminated manuscripts specifies 1 part white lead by volume to 3 parts slaked plaster, plus a little honey and a tiny bit of hide glue.  Nobody uses that receipe any more because of its poisonous nature, but nobody has found a good replacement yet, either.

The plaster provides a firm surface for burnishing the gold leaf, the honey binds the plaster plus provides stickiness to keep the gold adhered, and the glue acts as additional binder for the plaster.  I'm guessing that the white lead acts as a sort of plasticiser, but I don't know for sure.  I can't imagine what other part it might be playing, if not that. 

I do know that without it, the gesso doesn't like being burnished after it fully dries: the leaf has a tendency to flake off again unless the gesso is loaded up with honey in which case the leaf won't burnish at all because the gesso is too hygroscopic. 

So without the lead, the trick is to do the burnishing at the point where the liquid (water, glair) has almost but not quite evaporated, so that the plaster has firmed up but still has some plasticity.

I've been thinking that powdered aluminum might add the same sort of plasticity that the white lead does.  Or if not aluminum, perhaps some other relatively non-toxic powder.

Thoughts?

Offline Rabn

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Re: Replacing white lead in gesso sottile
« Reply #1 on: May 09, 2008, 08:24:00 PM »
the periodic table is your best friend in questions like this.  Tin is in the same column as lead, but is probably hard to make white...what is the chemical formula for white lead? Is the whole surface being covered with gold leaf?  If so, who cares what you stick it with; I suggest Shoe Goo or PlastiDip. I obviously have no idea what you're talking about, but that could change very quickly.

Offline Valdorod

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Re: Replacing white lead in gesso sottile
« Reply #2 on: May 09, 2008, 11:32:08 PM »
White lead is Lead (IV) oxide, the famous white pigment in paint, (ie lead paint).  titania (TiO2) is what has replaced white lead in almost all paints, completely non-toxic, it is even used as a pigment in foods and cosmetics. 

One of the few areas where titania has not replaced white lead has been in the shipping industry, lead paint is still better at ressisting corrosion.

You can use Titanium dioxide to replace the white lead in your recipe.  There is a company that sells a commercial form of guesso called liquitex, that uses titanium dioxide.

Valdo

Offline Katzenjammer

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Re: Replacing white lead in gesso sottile
« Reply #3 on: May 10, 2008, 02:35:44 AM »
Is the whole surface being covered with gold leaf?  If so, who cares what you stick it with; I suggest Shoe Goo or PlastiDip. I obviously have no idea what you're talking about, but that could change very quickly.

The fact that white lead is white is certainly helpful, because the substrate is visible through the leaf. Most gilders use a touch of Armenian bole or similar to impart a reddish color for that reason.

The gold doesn't cover the whole surface; it's usually applied to major letters/words or their backgrounds, or to form foliage decoration (e.g. ivy)  in the margins of the page. 

If just making it stick were the only problem, using more honey or even, as you say, shoe goo or something else would be fine.  But the ultimate goal is to polish the gold to a high brilliance, so that it catches and reflects the light to the viewer.  Unless the gold can be polished that way, there's no point in using it at all.

Offline Katzenjammer

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Re: Replacing white lead in gesso sottile
« Reply #4 on: May 10, 2008, 03:41:39 AM »
White lead is Lead (IV) oxide, the famous white pigment in paint, (ie lead paint).  titania (TiO2) is what has replaced white lead in almost all paints, completely non-toxic, it is even used as a pigment in foods and cosmetics. 

One of the few areas where titania has not replaced white lead has been in the shipping industry, lead paint is still better at ressisting corrosion.

You can use Titanium dioxide to replace the white lead in your recipe.  There is a company that sells a commercial form of guesso called liquitex, that uses titanium dioxide.
Liquitex makes a respected line of acrylic artist's paints, including a "gesso".  But while their product can be used for preparing surfaces for acrylic or oil painting,  it's not really useful for gilding mainly because it has no quality of hardness.  It doesn't respond to burnishing except by breaking up. 

Titanium dioxide was one of the first things tried as a substitute for the white lead, probably for the same reason you suggest it.  But while it's now the standard replacement for white lead as a pigment in paint and works pretty well (not quite as opaque or "buttery"), pigmentation must have been only a secondary role for the white lead in gesso because people have been fiddling with different proportions of titanium dioxide for years now without noticable success.  I had high hopes for it years ago myself, but I honestly can't detect much real difference in the receipe whether I put it in or leave it out.

I have no training in chemistry at all, so my ideas might seem naive.  I thought that paint tubes were still made of lead foil but discovered that they're apparently made of aluminum these days.  So that's when I started to wonder what exactly the white lead was contributing to the gesso and whether aluminum might not, at pigment-level granularity, substitute for it. 

My original thought was to use aluminum oxide, since it too is white, but then I discovered it's a high-class abrasive so I thought that might not be such a nifty quality to impart to the gesso.  So I was back to thinking about plain aluminum powder. 

I do a little work with metals, mostly silver and copper, in connection with jewellery and enameling, so I'm used to thinking of metal as being fairly plastic - I use a steel burnisher to remove scratches, for example, and harden or soften a piece with hammer or fire.  So it seems natural to imagine soft, powdered aluminum "loosening" the gypsum crystals enough to plasticise the gesso compound.  But aluminum powder is expensive and (apparently) dangerous, and gesso takes a month to make, so I thought I'd get some advice from real chemists before I leaped in and maybe wasted time and money.

Offline billnotgatez

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Re: Replacing white lead in gesso sottile
« Reply #5 on: May 10, 2008, 04:05:24 AM »
As you have seen the plasticity of various metals is different. My guess is that aluminum is more like titanium than lead. Lead and gold are similar. Finding a good substitute that is not toxic would be interesting.

Offline Katzenjammer

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Re: Replacing white lead in gesso sottile
« Reply #6 on: May 11, 2008, 04:54:17 AM »
What about tin dioxide SnO2 (coming off Rabn's mention of tin)?  It's a whitish powder, insoluble in water, and not especially toxic, though tin is allegedly (and counterintuitively) dangerously reactive and flammable according to http://periodictable.com/Elements/050/data.html. 

Is there a way to find out SnO2's mechanical properties as compared to white lead?  I'm looking at things like Young's modulus and bulk modulus because they seem from their descriptions as though they might be relevant, but I'm only guessing.
« Last Edit: May 11, 2008, 05:16:47 AM by Katzenjammer »

Offline billnotgatez

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Re: Replacing white lead in gesso sottile
« Reply #7 on: May 11, 2008, 10:56:51 AM »
Tin Oxide is considered a food safe colorant for glazes in pottery. But, it is also considered expensive by potters.

Offline Valdorod

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Re: Replacing white lead in gesso sottile
« Reply #8 on: May 11, 2008, 12:16:44 PM »
From what you have described the property of white lead that you need to replace is its property as a plasticizer.  You have also mentioned that for several years many have tried different things unsuccessfully.  I do not think that replacing it with a pure metal will work at all the properties of the oxides are very different from the metals.  SnO2 sounds like a good idea, however, from your information I wouldn't doubt it if someone has already tried.

Perhaps, thinking a little outside the box and trying a natural or synthetic clay.  Bentonite is used as a plasticizer in clay bodies or for suspension in glazes.  I have worked with a synthetic attapulgite clay called attagel.  Attagel clays when added to paints and adhesives allows the material to be spread easily when rolled, sprayed or brushed.  It also allows the hardeding process to be somewhat controlled.

Valdo

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