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(brian baade)
Hi Koo
I just returned from a conference in Belgium and finished grading so I am
only now getting to your question. I will respond to each point within each
section of your question and in red.
Hi Brian,
In a reply to a recent post you said, "…works painted over earlier
compositions are much more likely to develop delamination issues over
time." This raised questions for me.
1. Why is this so? Is it because
newly formed oil paint films don't crosslink with dried paint
underneath? How much less absorbent is a dried oil paint film
versus acrylic gesso ground? And were you referring to oil
paint only, or other mediums?
It would be an oversimplification to make a definitive statement that covers
all situations, but I will try to make some general statements. I do not think
that it is an issue of the newer paint not “cross-linking” to the older layers
but another group of possible issues. First is the diminishment of absorbency
and tooth. This is inevitable. Additionally, if the underlying painting was taken
to a high level of completion the artist may also be have added additional
medium in their later layers exacerbating the points above. If there was a
substantial period of time between the first and second composition, there
could also be a certain amount of surface grime that could slightly compromise
adhesion. It is also generally true that the more layers used to execute a painting,
the more likely a problem can arise. Finally, there does appear to be some
closing off of the surface of a painting that occurs after the paint has dried
for a while. I have never seen this completely quantified but people have long
spoken of the presence of an extremely thin layer of oil that sits above the
surface of the pigmented layer. Perhaps due to a slight sinking of the
pigments. This may contribute to a closing off of the surface. Truthfully, I
have my doubts about this and its effect. I have, however, in my years conserving
old paintings. I have seen many examples of paintings that were overpainted in
oil by an unscrupulous restorer. Sometimes a very cautious and highly trained
conservator can this cleave this overpaint cleanly off of the original paint without
damage to the lower imagery. This does
suggest the lack of true adhesion between the original and the later application
of paint.
I am not sure that I would make a definitive pronouncement, but generally,
dried oil paint (that has not been overly thinned) would be less absorbent/toothy
than a properly formulated acrylic dispersion ground. Having written that, a good lean lead white oil ground can retain a good deal of absorbency..
The above is somewhat true of all mediums but there are very different degrees.
It is most pronounced with oil paint. It is much less or almost inconsequential
in acrylic dispersion painting where the medium is such a great adhesive. Egg tempera
probably lies between but my own experience with tempera would put it closer to
oil in the sense that the absorption greatly diminishes as layer upon layer of
paint is added. It is very difficult to apply broad washes early on without
getting overlaps while this is relatively easy after many layers of paint have
been applied.
2. Would you say the same is true for egg tempera; that
delamination is more likely to occur in works painted over earlier
compositions?
Probably, but it would depend on how many layers of tempera were used in the
original composition and how dry (absorbent) or greasy (unabsorbent) the surface is. There are just too
many variables to make a definitive statement.
3. The case is often made that dispersive adhesion is primary; however in my
experience with egg tempera mechanical adhesion seems equally important. When
I've continued painting on aged egg temperas (from a few weeks to over a
year old; i.e. partially or fully polymerized surfaces) the paint is more
difficult to work with; much more prone to lifting if I do things like sponge
on watery paint, or lightly sand or polish as I develop layers. This
seems to indicate less than ideal adhesion between old and new egg tempera
paint layers; and that they "are much more likely to develop
delamination issues over time". And yet I've also been told (by
well-informed people) that it's fine to paint atop
old, polymerized egg tempera with fresh tempera (as long as the surface is
clean). Your thoughts? .
You need either chemical or mechanical adhesion, preferably both. There may
be a similar skin that forms on the surface of the egg tempera as it oxidizes
over time. There are really a ton of oily components in egg yolk. This would be
a great research project. I can’t say with authority but you, and my experience
suggests that there is a diminishment of adhesion. I suspect that most of it is
the loss of mechanical tooth/absorbency.
4. To address the less than ideal working properties of fresh
tempera paint applied to a polymerized surface, I do three things to the
surface: 1. Wipe off dust, 2. Do a gentle sanding with a 1500-grit sanding pad,
to open the surface, 3. Apply a very thin nourishing layer of egg yolk medium
(1 part yolk to maybe 8 parts water). I'm actually not quite sure
why I do #3, except that it seems to help the paint grip and behave better –
but I also wonder if the nourishing layer is more detrimental (i.e. contributes
toward fatty acid migration, or even delamination) than
helpful. Again, your thoughts?
I would probably err on slightly roughing up the surface over adding egg
water. The later probably does help but adding a bunch of the adhesive (binder)
which by the very nature of its stickiness will adhere to the lower paint and
its freshness allows adhesion to the upper layers. It does, however add a slightly
slick interlayer, and more importantly, contributes a superabundance of free
fatty acids, which will possibly/probably effloresce from the surface in the
future. We have seen this on many 20th century egg tempera
paintings.
Brian
Thanks as always!
Koo
