Varnishing Icons
Icons have tempera and gold leaf on the surface so it is an option to put a protective layer on them so this is what happens when you do.

An Icon with extensive punchwork. Copyright (c)2020 Paul Alan Grosse Background
Icons have been around for over a thousand years and come in a number of sizes. The smaller ones are meant to be held as opposed to hanging on the wall or be part of a larger installation.

The construction is basically a wooden support, usually with a carved out depression although it can have sides built onto it, which has then had linen stuck to it which has then been gessoed.

Ignoring for the moment oil-painted icons and focussing on tempera-painted icons, this surface then has either: bole applied to it and then gold leaf which can then be punched; or, it has tempera painted on it. Either way, its surface is going to be fairly delicate and needs protecting, either: by taking care of it; or, by adding a protective layer to it.

Small icons that are meant to be held are usually protected by wrapping them in cloth and keeping them in a safe place.

Small to medium sized icons that are produced for putting on the wall can be protected with a layer of glass held in place by a frame but for larger icons, that is not necessarily feasible.

Many people who produce icons put a protective layer such as a varnish or linseed oil on the surface to protect them but that modifies the way that the pigments reflect light so to what extent does that happen and how does that affect other things such as punchwork?

Paint sample on gessoed linen covered oak. Copyright (c)2019 Paul Alan Grosse On the right, you can see the sample that I produced. The photographs were taken using a colour bar so that the colours could be corrected for any variations in lighting conditions and so on.

You can see the four blocks of colours over witch I applied three different varnishes, leaving one as the control.

Each block of paint was applied over a ground that was left plain for the upper half and had black paint over the lower half so that the level of transparency can be evaluated visually and numerically.

Paint sample on gessoed linen covered oak. Copyright (c)2019 Paul Alan Grosse And this is it after it has had the gold and the varnishes applied.

Painted on top of gesso, which has been applied to linen covered quartersawn oak, the areas of paint straddle bare gesso and a band of lampblack paint so as to show up any transparency in the raw paint and also to see if that level of transparency has changed when the varnish has been applied.

Also, you can see the extent to which, if any, the varnish has altered the punchwork.

You can see from the picture above that the areas with paint in are one a few millimetres across.

The colours are as follows:

 Paint sample on gessoed linen covered oak. Copyright (c)2019 Paul Alan Grosse
1.
24 kt Gold leaf on
Armenian Red Bole		 2.
Dutch Lead White
(Home Made from lead roofing sheet)		 3.
Raw Umber
(Cornellisen)		 4.
Red Ochre
 
5.
Armenian
Red Bole		 6.
Cinnabar
Russian		 7.
Red Lead
Cornellisen		 8.
Lead Tin Yellow I
Russian		 9.
Yellow Ochre
Cornellisen		 10.
Gold Ochre
Russian
11.
Orpiment
Russian		 12.
Glauconite (Terre Verte)
Russian		 13.
Malachite (fine)
Inoxia		 14.
Malachite (coarse)
(Home Made)
15.
Azurite (Coarse)
Russian		 16.
Lapis Lazuli
(Cornellisen)		 17.
Verdigris
(Home Made)		 18.
Lamp Black
Cornellisen
    Control linseed Oil Colophony Winsor & Newton Gloss Varnish
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  The control here is really just to show that the
two photographs of the sample don't vary
too much once they have been corrected.		 The increased transparency of the white together
with yellowing of the white and darkening
of the red ochre but especially the umber.		 Not so much yellowing but there is darkening and
increased transparency of the darker colours		 Increased transparency of all of the colours here.
The gold has remained all right throughout
these samples, along with the punchwork.
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  A bit of transparency overall but the yellow and
gold ochres seem to suffer the most.		 Most of the levels of transparency are pretty
similar but the red bole seems to darken
the most.		 These results are pretty similar to the linseed
oil sample.		 The red bold darkens quite a lot with the varnish.
    Control linseed Oil Colophony Winsor & Newton Gloss Varnish
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d		 Tempera varnish paint sample varnished Tempera varnish paint sample varnished Tempera varnish paint sample varnished Tempera varnish paint sample varnished
  These all have fairly good covering power once
there is enough of the pigment there.		 The orpiment and coarse malachite transparency
increase slightly but the glauconite darkens
considerably and the fine malachite becomes
almost completely translucent.		 Like the linseed oil again but not quite as bad. Again, like the linseed oil but not as bad as
the colophony.
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l		 Tempera varnish paint sample plain Tempera varnish paint sample plain Tempera varnish paint sample plain Tempera varnish paint sample plain
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  The lamp black was put on thinly so that transparency
could be seen. The verdigris is transparent any way.
The other two hide the contrasting ground well.		 The black becomes blacker and everything else is now
transparent.		 This is almost as bad as the linseed oil. This is as bad as the colophony.
  Control linseed Oil Colophony Winsor & Newton Gloss Varnish
 
Overall, the linseed oil - which icon makes who do coat their work will tend to use - has the worst properties of increasing transparency and darkening colours, especially colours that are dark to start with.
Additionally, it adds a yellowness to the pigment which can be seen the best in the lead white sample above.
Not quite as bad is the colophony - typical of natural resins and the Windos & Newton varnish is almost as bad.
The best looking is by far the untreated sample but of course, the cost of this is that the work needs to be protected in some other way.
At least we know now.
 

Diagram of tempera-painted icon showing light travelling through the pigment and being reflected by it. Copyright (c)2020 Paul Alan Grosse This is what happens when light travels into the tempera medium. The tempera medium teds to be just a framework that holds the pigment particles in place - the interstices within the tempera medium allows light to be reflected back to the viewer

Diagram of tempera-painted icon showing light travelling through the pigment and being reflected by it. Copyright (c)2020 Paul Alan Grosse This is what happens when a medium fills up those voids. An increased amount of light continues travelling through the paint layer and is reflected off the ground.

If there is detail that was hidden before, it becomes more visible now the transparency of the paint has increased so underdrawings, changes in the painting and so on become more visible.

In Summary

There are a number of factors that influence the transparency of a tempera when it has some sort of other medium added to it.

One of them is pigment particle internal reflection. This is largely down to how similar the refractive indices of the pigment particles and the medium are. If there is a lot of difference, then light is largely still going to get reflected within the particles. However, it the refractive indices are very similar, light will tend to continue without being reflected internally and as a result, the visibility of lower levels of the paint layer will increase.

The result of similar refractive indices is that the paint becomes more transparent.

The other factor is the transparency of the pigment itself. One particularly transparent pigment is verdigris. You can see that even without adding an extra medium to the paint, it is already transparent as you can see the different colours for the paint over black and over white backgrounds.

So, if you are going to put oil, varnish or resin on your tempera-painted icon, you need to be confident that the increase of pigment transparency that doing so will produce will not allow unexpected details below the surface to be revealed. Once you have put varnish on it, there is no going back to make adjustments. If you are using hydrophobic media such as: resin, you will only be able to use resin-based paints; and, for oil, oil-based paint. Adding a hydrophobic medium to the previously hydrophilic medium will take away your ability to add hydrophilic media.

All images and original artwork Copyright ©2019 - 2020 Paul Alan Grosse.