Hi. Thanks for the kind words. I will respond within
the text and in red
I've searched here on the topic, and also read the "rigid
supports" document in Resources (which is a wonderful reference), but I
have a few additional questions about working on copper.
1. I'm using relatively thick copper (14-gauge etching plates) and working
fairly small (maybe 11x14" or so, down to 5x7"). Is it necessary to
brace supports of this size, or would it be sufficient to put a lightweight
(floating) backing board behind the plate in the frame--perhaps Gatorfoam, or a
layer or two of museum board?
I do not think that bracing would be important or
even a good idea in that case. What you propose sounds fine and is exactly what
I have done in the past. It may be a good idea to put an acid free interleaf
between and board that may be acidic or become acidic to prevent corrosion of
the reverse of the copper panel.
2. My process to prepare the surface of the plate: degrease with
denatured alcohol, thoroughly abrade the surface with sandpaper or steel wool
(with the aim of completely stripping the surface to expose fresh metal),
vacuum off any copper dust, then degrease again, making sure that all dust and
residue gets removed. Then allow to dry, and prime. Does that sound about
That sounds good.
3. How about where the copper is not covered by primer/paint? Like the
back and sides? Can I just let that oxidize, or should I seal it with
something? Renaissance Wax, maybe?
I do not believe that most 17th century oil
on copper panels were not coated on the back. There are even a few instances
where the work was done on the reverse of an etched intaglio plate. However, if
you wanted to avoid corrosion on the back you could spray it with an epoxy
coating after degreasing the reverse. Do this before treating the front so that
your degreasing and abrasion of the front would remove any accidental overspray
that may have gotten onto the face of the panel.
4. I assume any oil-based primer will work? How about an alkyd primer, like
Winsor & Newton's Oil Primer? I have a few small test sheets on which I
tested some straight lead carbonate in linseed oil (RGH), a lead painting
primer that contains some titanium white and driers (Rublev), and the Winsor
& Newton primer. All three dried very quickly (the W&N primer was touch
dry in a matter of hours). I'm guessing that the copper is a drying catalyst?
The Rublev primer turned very green upon drying; I don't think that it was a
matter of surface prep, because the RGH lead primer is right next to it on the
same sheet of copper, and it didn't change color at all. The Winsor &
Newton primer took on a slight green tint, but it's barely noticeable.
Is this sort of thing common? Something to worry about? I don't think I'd use
the Rublev primer, since the color change in that case was considerable.
For me, priming copper panels has had a bit of voodoo to it. I
have primed a group of them at the same time and using the same prep and
materials and had one of the bunch turn green as well. These should be cleaned
of their ground and the process repeated as such immediate corrosion is a sign
of trouble, which would likely only continue if left in place. Copper can
catalyze drying oils but I am also guessing that the primers contain enough
driers to create a rapidly setting ground. I am unsure why the primers all preformed
so differently. Perhaps they employed oils of very different acid numbers. It
makes sense that an oil ground would be formulated with an oil of a higher acid
number as this tends to make a paint that contains a greater percentage of
pigment but one that yellows a bit more strongly. The leaness would be useful in
a ground and the yellowing would be far less of an issue than in a paint used
for surface effect. Again, all of this is conjecture.
If you have the time, it would be useful to make a
test of the available grounds and let them really oxidize for a while and then
test for scratching before deciding on your preferred ground. I have always
used lead white in linseed oil without a drier. There may be better modern
materials for this but I have not personally tested them. I know that there has
been a good bit of experimentation with various coatings on aluminum panels. I
will send this along to a couple of our other moderators to see if they have
anything to add.
5. I skipped the oft-recommended garlic step, just on the basis that I have
been able to find a consistent or empirically supported reason for its use.
Some sources say that it helps to "etch" the metal (though it is
unclear how, since garlic is not acidic). Some sources say that it might serve
as a wetting agent (which makes more sense, except for the fact that copper
doesn't seem to need a wetting agent--it takes oil paint really well, with no
beading). Some sources say that it helps the paint bind to the surface
chemically, rather than just mechanically, but I don't think that's correct.
Don't oil films exchange ions with copper? In any event, the idea of putting an
aqueous paste between the metal and primer seems like a bad one to me, but
perhaps there is a purpose for this step that I haven't considered?
I do not think that garlic is necessary for a stable
panel. Garlic is not very acidic but a cursory internet search shows that it
has a pH of around 5.8, which is well below neutral (7). However, I do not believe
that etching plays a major role here no matter what. The dried garlic juice does
create a slightly pebbled or textured surface that may promote adhesion.
whetting, I have personally seen thinned lead white ground bead on a copper
panel that did not have such a layer. This may have just been an isolated occurrence
or perhaps that panel was not sufficiently abraded.
I have discussed this idea with a fellow conservator
who is more science savvy and they suggested that the garlic may also perform
an additional effect; promoting a more stable form of corrosion. The copper is
going to corrode to some degree at the interface between the panel and the oil
primer (perhaps there are modern materials that would not do this) All
cross-sections of paint taken from oil on copper paintings that I have seen
exhibit a green layer at the bottom of the oil priming. The sulfur in the thiol
groups contained in garlic may promote the creation of more stable corrosion
In closing, though, I do not believe that the garlic
application is necessary nor does it absolutely make a more stable panel, I do believe
that people did believe that it did, and that this was because of some of the aforementioned