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Subject: Lacquered papier mache tabletop

Lacquered papier mache tabletop

From: James Hay <james_hay>
Date: Thursday, November 9, 2006
Ruaidhri O'Bolguidhir <ruri [at] eircom__net> writes

>The varnish on the surface is very stubborn and solvent tests
>carried out have shown that dichloromethane is the only effective
>solvent. Even dichloromethane is difficult to use because it
>evaporates so quickly. Does anyone know of a way to gel
>dichloromethane which would allow me to apply it to the surface with
>more control?

In North America, you can obtain dichloromethane already gelled with
paraffin, prepared at a commercial scale and thus available in cans,
as "Polystrippa", a carcinogenic and poisonous, but very effective,
paint remover sold in every hardware store. Over here, the active
chemical is usually called methylene chloride, but the two names
describe exactly the same chemical, a methane base with two chlorine
atoms substituted for two hydrogen ones, that is, CH2CL2.  It works
dangerously fast, and is also just plain dangerously toxic. As well,
it doesn't offer a strong scent, so there is no odour warning before
you've overdosed, which leads to heart attacks. But, if you work
carefully and with particularly good control of ventilation, perhaps
you will be able to use it. Think ahead: A few thousand swabs later,
you'll want your health to remain intact.

Mind that there are plenty of other slower, weaker, and perhaps less
poisonous chemicals to use to swell, soften, and remove paint or
varnish. Talk to the paintings conservators, as they seem to be the
ones with the most experience dealing with recalcitrant varnish.
When we furniture conservators deal with varnish, for the most part
the varnish is sitting on top of wood, a material reasonably
resistant to damage from solvents, except of course to that
ubiquitous, relatively non-toxic, and carbon-free solvent, water.

If the only material that shifts your varnish is dichloromethane, to
me that signals that the varnish has polymerized. If it has
polymerized, then you are not going to be able to 'reactivate' the
varnish, like you might be able to do with a 'spirit' varnish. The
chemistry of the varnish has changed since it was applied, exactly
what you would expect from an "oil" varnish, and your solvent tests
have revealed precisely that.

I hope to hear from the DistList that, in fact, someone has
discovered how to soften, remove the obscuring dirt from,
re-amalgamate, and flow out to a smooth, transparent, piano finish,
the wide range of disgustingly dirty old ruined finishes that one
may see on old furniture. Please do speak up if you have that news.
But unless I hear differently, I think the case is that, barring a
wonderful new discovery in the world of chemistry--something we
could all hope for--'de-polymerization' that could lead to
re-amalgation of the original finish is not yet possible. If you are
determined to remove the obscuring darkness of a polymerized varnish
film, I imagine your remaining options are to soften and remove it
chemically, if cautiously, or slowly grind through it to the design
layer.

We were recently considering what to do with a very dark, almost
opaque varnish over wood. We cleaned the surface, hoping for a big
improvement in the ability to see the veneer beneath, but cleaning
had almost no effect on transparency. All that happened was that the
surface was revealed to be shiny and reflective, once cleaned. Our
finish was quite transparent, not pigmented, but still very dark. We
had samples of the finish analyzed and discovered that the material
was made of linseed oil and colophony, that is, pine resin, with no
trace of particles indicating the inclusion of pigment.  Perhaps
your analysis will reveal to you that you have a similar resin mix
on your table. We had the luxury of being able to justify leaving
the film in place, and that is what we did, but I don't foresee that
route as being one of your options. There's your challenge, eh? Best
wishes.

James Hay
Senior Conservator
Furniture and Decorative Arts
Canadian Conservation Institute
Ottawa, Ontario K1A 0M5
Canada


                                  ***
                  Conservation DistList Instance 20:26
                 Distributed: Monday, November 13, 2006
                       Message Id: cdl-20-26-002
                                  ***
Received on Thursday, 9 November, 2006

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