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MITRA Forum Question Details

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  • isolating coat over egg tempera grisailleApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2020-02-18 15:25:41 ... Most recent comment 2020-02-19 09:00:07
    Varnishes Egg Tempera


    I have serious doubts about what kind of isolating coat would be most appropriate for me.

    I'm looking for a varnish to seal egg tempera that is also compatible with a final varnish, which will go over the last layer of oil.

    As the isolating coat penetrates over the tempera, it becomes part of it. I would like to know, according to your criteria, which combination of varnishes I could use.

    Here is the structure of my paintings:

    Egg tempera underpainting in grey values. This layer is extremely matt, due to the fact that I thin the ET with a lot of water. Then I apply combined oil glazes and intensify the lights with white tempera grassa (highlights). Finally I finish the painting with oil paint.

    I have tried Paraloid B72 (15%) by Kremer and it is completely transparent. The problem is the brushability. So I only manage to apply it in small areas. It is impossible to give a uniform layer to the whole painting and I prefer not to spray it.

    I have tried Gamvar (Gamblin) as well. The brushability is great, kind of gelatinous. But even after drying it is still a bit yellowish or dark. And I am not sure if it is compatible with ET.

    One last question. In the following link it says that if adding UV light stabilizers to the varnish it should be added to both layers (isolating coat and final varnish), what do you think?

    Thanks a lot in advance and best regards.


Answers and Comments

  • EditDeleteModerator Answer

    The recommendation from the article on the Natural Pigments web site and what I originally provided here are for an isolating varnish and a final picture varnish over egg tempera painting—not between egg tempera and oil painting layers:

    All varnishes work differently on casein, distemper, egg tempera, gouache, and watercolor paintings than on acrylic and oil paint because the varnish is absorbed by the paint and/or paper, becoming an integral part of the picture that could cause discoloration as the varnish ages. In addition, varnishes on these paintings such as egg tempera cannot be removed because they become an integral part of the paint layer.

    If egg tempera is to be varnished, an isolating coat is necessary to allow the application of a final picture varnish that can be later removed once it becomes dirty, yellow and brittle.

    UV hindered amine light stabilizers can extend the life of both a final picture varnish and isolating varnish, so it is desirable to have it in both varnishes.

    The main problem with most varnishes on egg tempera is haze forming under the varnish after it is applied to the painting. We believe that varnishes made with polar solvents are less susceptible to forming haze than those with aliphatic and aromatic (non-polar) solvents. We are presently testing such varnishes on egg tempera paintings.

    George O'Hanlon
    2020-02-18 15:43:46
  • EditDeleteModerator Answer

    ​I do not recommend applying any resinous varnish over an egg tempera painting to isolate it from oil paint, because of its solubility should the oil painting be varnished or cleaned using solvents after it is completed. The recommendation above is solely for an egg tempera painting.

    George O'Hanlon
    2020-02-18 16:01:46
  • EditDeleteModerator Answer

    I do not recommend resinous coatings under oil paint for the reason George mentions but also because the resin can become incorporated into the superimposed oil paint if it is soluble in mineral spirits. The presence of a UV scavenger could make this even more problematic as if this were to become incorporated into the oil paint it could severely inhibit the oxidation and curing of the paint film.

    The way around that is to use a varnish that is not soluble in the same sort of solvents that are used to thin paint (shellac, B-72, etc) The problem here is that the isolating coat could still be attached by slovents used during later conservation treatments. It is not uncommon for alcohols and acetone to be apart of a cleaning mixture. If you decide to do something like this make sure to record your practice and materials on the back of the artwork (see our “Resources” Section)

    Sorry that was not of more help.

    Brian Baade
    2020-02-18 16:27:29
  • ApproveRejectUn-ApproveUser Comment

    Hello All,

    In my experience, isolators with quickly evaporating solvents work best (shellac, B72) because slow drying isolators have time to sink into the porous, absorbent surface of egg tempera; however a fast drying isolator does not have any brush-ability, so they're hard to apply consistently over a large surface (the largest size panel I've successfully applied a coating of shellac to was perhaps 14 x 24, and it was tricky to do well). There's also the challenge of these isolators being dissolved by later conservation efforts, but if you are careful about labeling your paintings, as Brian suggests, you've mitigated this issue. 

    Slower drying isolators can be brushed out smooth; however they're more apt to sink into the paint layers (depending on how developed the egg tempera is) and thus be very slow drying; and their solvents have a tendency (in my experience) to draw out lipids that eventually create the efflorescent haze George mentions.  I've tried Gamvar to isolate tempera - it sunk in and was very slow to dry, for me; and eventually I got some haze (as I have with most non-polar solvent based isolators).

    I'm not a solvent expert, so I need Brian and George's help here.  Paraloid B72 is also soluble in Shellsol A, which makes it brushable – but I'm pretty sure Shellsol A is a non-polar solvent, so it's doesn't solve the problem – yes?

    There are also substances that can be added to shellac to improve its brush ability.  See attached documents, from Shellac Addtitives 1.pngShellac Addtitives 2.png I've never tried them, don't know what they are (polar, non-polar).  I also appreciate the issues conservators point out about shellac (although, personally, I use and like a platina, de-waxed shellac for isolating).

    One final thought: Max, you say your process begins with an "Egg tempera underpainting… this layer is extremely matte, due to the fact that I thin the ET with a lot of water." Tempera is a naturally more matte medium, but if you are properly tempering (correct ratio of egg yolk to pigment), regardless of the water content (which evaporates out), I don't think a thinned paint should result in a surface that's more matte than usual. What do you think, Brian and George?  I just don't want Max to be under tempering his initial layer. 

    Koo Schadler

    2020-02-19 09:00:07

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