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On Varnish

By Karen Thomas - 05. June 2022
Getting beneath the surface of Old Master paintings in the studio of Karen Thomas.
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In the Studio with Karen Thomas

Part conversation, part handbook, our In the Studio series visits conservators, framers and other craftsmen in our field to explore what it takes to look after an Old Master picture. For the first installment, we met with Karen Thomas in her Flatiron District studio to investigate what’s behind the finishing coat.

Karen credits Robert Hellenga’s novel The Sixteen Pleasures — in which an American woman travels to Florence after the 1966 floods to restore damaged books and ephemera — for her interest in conservation. During graduate school, Karen discovered she preferred to work on pre-modern rather than contemporary paintings, explaining: “40 hours of retouching a field of blue was a little hard.” She worked as a conservator of European and American paintings at the Metropolitan Museum of Art in New York for almost a decade before starting her own practice, which is now in its tenth year.

Let’s begin with a basic question: what is varnish?

The traditional artists’ varnish, which we still use today, are made from tree resins. And the two most common are mastic and Damar.

If we’ve varnished recently in the studio and people walk in, they will mention the smell, which we don’t notice as much. We work with it! The smell comes not from the resin, but from whatever solvent that it’s dispersed in. And we often use turpentine, which is a traditional paint thinner, and it’s a smell I really love!

Why is it necessary for artists to apply varnish to a painting? 

The varnish helps provide an even gloss to the painted surface. It also helps saturate the pigments, to make sure they look the way the artist intended them to: the darks really look dark, the colors look properly saturated. The varnish also provides a protective layer to the painting – it’s a sacrificial layer that artists knew long ago that could come off and be replaced.

Could you introduce, for a newcomer to Old Masters, how an artist would’ve painted between the 14th and 19th century?

First, the artist would have a choice of what they’re going to paint on. The three most common would be a wood panel, a canvas support (woven fabric, often linen), or on a thin copper plate. And all of those would have to be prepared with a ground material, which is usually a white layer of chalk and glue that would be painted, or rubbed, onto the surface. Later on, artists start to use colors – for instance, you’ll get red grounds in Southern Italy, and sometimes grey grounds in the North.

Then, in the case of oils, the artist would apply oil paints. They would grind the pigments into the oil and make them into a paste that they could paint with: so you have oil, which is the binder, and the pigments, which are the colors – that’s the paint!  Once the painting is done, different paints will dry with a different sort of sheen to them, because different pigments need different amounts of oil.

Some Old Master paintings were not painted in oil. Its use only became widespread after the mid 15th century, and even then, artists like Botticelli were slow to switch from tempera, which uses egg as the binder for pigments. Did tempera paintings also require varnish?

Tempera paintings need varnish as well, though they’re a little bit less in need of saturation in the same way that oil paints are.

What is the most common problem with varnish?

Both Mastic and Damar will turn yellow over time.

How does one identify discolored varnish?

It’s not always easy. But for someone who’s just starting out looking at paintings, it’s easiest to look at something that should be blue or white. So, if you look at the sky of a painting, and it seems a little bit greenish, that’s probably an indicator that the varnish has discolored. If you’re looking at a painting that doesn’t have a sky, look for a white highlight and see just how yellow it looks.

Is there an example of discolored varnish we can see in a museum?

The most obvious would be the Mona Lisa. That is famously discolored, and they will probably never clean that painting. So, it’s always going to have a yellow varnish.

Mona Lisa © Alamy / Mihai Barbat

A few years ago, you worked on a Carlo Maratti painting from the Odescalchi collection, which had a very dirty varnish when it was found in a public auction. The picture now also belongs to a museum. What happened after you took off the old varnish?

I used two different varnishes – Mastic and Damar. When I’m restoring a painting, I always put a varnish on before I do any retouching because it separates any material I’m using from the original paint. I chose Mastic for that first layer because you can use Mastic in a very thin layer and saturate the pigments. So that makes it easy for me to retouch, because in the end, I would like to have the least amount of varnish on as possible. Afterwards, I used Damar to seal the retouching and provide an even gloss. It has slightly different visual qualities to Mastic, and I wanted a little bit more of a gloss to it.

Carlo Maratti, Sacrafice of Noah, sold by Nicholas Hall to the David Owsley Museum of Art at Ball State University, Muncie
How do you choose between a glossy or matte effect?

To a certain degree, the amount of gloss depends on personal taste. To have a strong gloss on a painting was a trend that has come and gone. I personally don’t love it, but if an owner asked me to make a painting glossier, I would.

There are also reasons you would want to have very little gloss, and that is usually because you’re trying to minimize the appearance of any texture on the painting. So, planar deformations on a panel will be much less visible in a normal viewing environment if you have a lower level gloss on the painting.

But you can also make some generalizations. For instance, a Northern panel painting could go with a little bit more of a gloss to it. Whereas if you put a high gloss on a Southern Italian painting with a really strong weave, you’re just going to get reflections off the weave.

How do you apply the varnish?

You can brush it on with a large brush, or you can spray it on. We have a special spray gun that applies it in a very fine mist. Sometimes you get a little bit of texture with this spray, which you may want to accentuate. Or we may spray because you can’t brush the varnish on for various reasons. But usually, it’s because of its visual effects that you would choose one over the other.

While working on a group of Pontormo grisailles for an exhibition at Nicholas Hall last year, we learned that one of them had a layer of tinted yellow varnish to accentuate the aged effect. Is this something you come across often?

Tone varnishes are very common, more than people realize. Sometimes a painting is tinted with varnish overall, and sometimes only part of it is coated. There are two main reasons this would’ve happened. One is because in the 19th century, there was a real interest in the golden glow’ of the Old Masters, which came from the paintings having a discolored varnish. So after they cleaned the painting, they wanted to replicate a discolored varnish. The other reason is that using a tinted varnish is an expedient way of disguising wear and abrasion or thinness in certain passages, especially in browns and blacks. So, if you just wash a tinted varnish over those areas, it can pull it together very quickly. On the other hand, it can soften a lot of the details and darken anything that’s light. It’s not beneficial, so it’s out of fashion now.

Partial removal (left half) of darkened varnish: Jacopo da Pontormo, Cupid and Apollo, 1512-14, oil on canvas. Samek Art Museum, Bucknell University, Lewisburg © The Conservation Center, Institute of Fine Arts, NYU and Nicholas Hall

And are there other notable trends in conservation to do with the varnish?

I’ve noticed a move from natural resin varnishes to synthetic varnishes. That happened around the 1940s, but people are shifting back towards the natural resins now. I think it’s much more common for people to return to, particularly Damar and less or so Mastic, because the aging properties of the synthetics are not what they hoped they would be.

Can you tell us a bit more about synthetic varnishes?

People often think about discolored varnish as yellow. But a lot of paintings that you see on the market have synthetic varnishes, particularly at auction, and they tend to turn gray and cloudy. The colors are not necessarily as affected as they would be with a yellow varnish, but the contrast would be. Your darks are never going to look completely dark, and your lights will look darker than they should. When you have that issue, it can make a significant difference to take a varnish off.

For example, if you’re looking at a Dutch Old Master portrait, in which the subjects often wear black garments, you might see that nothing looks completely black, nor do the whites look bright. It doesn’t look like discoloration, but it’s sort of flat. Then you probably have a synthetic varnish that’s aged poorly and should be removed.

What does it mean for a picture to look ‘flat’?

The contrast is part of what creates a sense of depth in a painting. So when your darks are not completely dark, you lose the sense of space within a painting. Sometimes people will joke that it looks like a placemat rather than a painting!

How does a collector determine whether or when to have a work cleaned?

My approach is pretty conservative. Even though it’s what I do for a living, I always encourage people to put off cleaning as long as they can. But if it’s really affecting the way the painting is supposed to look – the space doesn’t read quite right, colors are off – then it should be cleaned. Also, if other work needs to be done on it, the retouching has gone off, or there are structural issues, then you will probably have to clean it in order to address those issues. But if it’s just a little bit yellow, and you can live with it, you should. You’re exposing the painting to solvents every time it’s cleaned, and you want to limit that exposure over time.

Are there ways of slowing the discoloration of the varnish? 

We now have materials that we can add to the varnish to delay the discoloration. So while a varnish used to discolor in 40-50 years, now it could last, in the best case scenario, 70-80 years.

Is varnish alone sufficient to protect an Old Masters painting?

I worked on a painting in an apartment across from Central Park, where direct light shone through a very large window onto the painting. Within 10 years, the reds had faded, and it was a 400-year-old painting, at least. You definitely want to keep paintings out of direct light, and if that’s not possible, then you’ll need UV protective glazing, which you can get from any framer.

Are there other situations where you would recommend UV protective glazing? 

The other time you would think about glazing is if you have a situation where you can’t control the environment in terms of humidity and temperature. You can have a certain amount of variations, but it’s best if the changes are as slow as possible. Paintings on wood panels are particularly sensitive. If it’s going to hang in a place where the humidity in the winter is 10-15%, and in the summer above 50%, which is not uncommon in a city like New York,  you might want have a so-called climate envelope made for it (also called a climate package). It features UV glazing in the front and is sealed around the back to create its own micro-climate.❖

Karen Thomas is a paintings conservator at Art/Work Conservation Partners in New York.
Editor: Mary Wang
Media production © Joseph Joseph Studio
Music © Huizi Zhang
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