Tempera Grassa atop Oil?

[Koo Schadler]

On the subject of tempera grassa....There is a website devoted to Van Eyck's Ghent altarpiece:http://vaneyck.kikirpa.be/It allows for high magnification and you can get very close to the work. There have been writings about Van Eyck and others in the transitional period between egg tempera and oil which claim painters were using both oil and tempera, and/or tempera grassa - but it all seems rather fuzzy, how (or even if) they combined the different mediums. Looking at Adam in the Ghent altarpiece, his skin tones are smoothly blended (as they would be in oil) but on top of him sit innumerable, crispy rendered, fine hairs (very linear, as if done in tempera). As I was scrolling over Adam I came across (above the knee cap and below his fig leaf) hair lines that aren't crisp but instead have beaded up - i.e. like a water media applied atop oil. I'm presuming this is tempera or TG on top of oil? Any other thoughts on what would cause the paint to behave that way; or what mediums are in play here, in what order?Koo

[John Kennedy]

Koo, your link isn't working for me and so it would be hard to respond to your questions. See if you can get the right link up and I'll check back after I visit the site. Peace be with you. Blessings, John

[Koo Schadler]

Hello John,

I've neglected my postings for some time now, so pardon my tardy reply. Below, hopefully, is the correct link. In a book I'm reading about the National Gallery of London's collection, Jill Dunkerton, a well known conservator there, mentions that the Ghent altarpiece is all oil - but I don't understand why the paint would bead up in small strokes if it all were oil...? Any thoughts?


http://closertovaneyck.kikirpa.be/#home/sub=altarpiece

[Robert Meredith]

Koo: When you paint over a dried painting with an oily medium the oil will occasionally crawl or bead. This often happens with glazes. The surface tension of the paint is greater than its adhesion to the painting and it literally pulls itself apart. A line will separate itself into little dots of color. This is especially true on a very smooth surface where there is very little tooth for the paint to grab hold of. The lines on the Ghent Alterpiece that have beaded are probably oil that was added after the paint had dried. The fine lines on the head are no doubt egg tempera or tempera grassa over the oil. Tempera on oil doesn't crawl. Tempera bonds very well to oil paint and you can execute much finer lines with tempera on oil than you can with oil. Oil has a soft edge while water based paints or emulsions have a hard edge. Egg/oil tempera and dry pigments are excellent for punching-up details, fine lines and highlights on a completed oil painting. Egg/oil tempera or tempera grassa can lean toward either oil or water according to the needs of the artist. The face of Adam could have been painted with an oil rich mixture while the fine detail could have been done with a mixture containing more water. Once the emulsion was mixed Van Eyck could have altered its composition on the palette simply by adding more oil or water with his brush as needed. Thank you for the Ghent Alterpiece address.

[Koo Schadler]

Hello Robert,

Thanks for your response. Your point is well taken that even oily paints may bead up on a dry and especially smooth surface. That could explain Adam's leg hairs.

I'm interested in your comment that tempera bonds well to oil paint. This has been a confusing subject for me. I generally look to three sources for information on painting: the scientific knowledge of chemists and conservators (I don't know much about chemistry; I count on George for that. :-)); the experiential knowledge of painters who have worked intensively in a medium for a long time; and historical/written references.

Regarding tempera over oil, the chemist and conservators seem to agree that pure egg tempera does not bond well to oil paint, an oil ground, or an acrylic ground. Pure egg tempera bonds best to true gesso or previous egg tempera layers, but not much else. At least, this is what I've always read and have been told. Experientially, I can say that I've worked in tempera for a long time and my experience affirms the above - tempera does not adhere well to oil. Regarding the written record, I have seen it mentioned that thin strokes of "tempera" paint can be "embedded" in a fresh oil glaze, as exemplified by highlights in the hair of some Renaissance portraits. (Unfortunately I haven't seen mentioned whether the "tempera" in such examples is pure egg tempera, or a tempera grassa that tends toward egg, or a tempera grassa that tends toward oil.) Given the above, you can see why I'm interested in your comment...have you had short and long term success with egg tempera over oil?

The reason this issue confuses me is that, while all that I've been told and my own experience affirm that egg tempera does not sit well a top oil, the binder in egg yolk isegg oil. Granted, the egg oil is emulsified in water (and water based paints don't generally behave on oil), but given that the binder itself is a type of oil (I had one conservator tell me he considers egg tempera a type of oil paint), why doesn't it behave well on other oil mediums? Is it because of the water vehicle? Or because egg oil is not as strong a binder as other drying oils like linseed, walnut, etc...? George, I'm hoping you can shed light on this.

Another question for anyone who has experience on this topic: do you think you could achieve as precise a line as is possible in egg tempera (or an egg-rich tempera grassa) using oil paint, if that oil paint had been modified (with resins, perhaps)? I sometimes wonder if the crisp linearity in work by, for example, the Flemish (so-called!) primitives does not come from either pure egg tempera or tempera grassa, but rather from those early oil painters' feeling for linearity (inherited from their experience with tempera), and their use of an oil paint that'd been amended with resins, which enabled the oil paint to be applied, when necessary, as "crisply" as egg tempera (along with, of course, those painters' great skill and patience!)

I've spent a long time learning how to make egg tempera appear (in parts, when I want) like oil paint, and feel now I can do so to some degree (beyond what many books say egg tempera is capable of). But I'm not enough of an oil painter to know if the reverse is true: if properly modified, and if the painter has sufficient skill and the intention, can oil paint be made to behave more or less like egg tempera (in its visible results; specifically, that crisp linearity for which tempera is renown)? I'd love to hear from anyone with experience in this (granted, rather singular) quest!

My uncertainty with the Ghent altarpiece is this: within the framework of my long experience with pure egg tempera, and my much more limited experience with tempera grassa (of both types) and oil, I don't see how Van Eyck could have gotten (a) such smooth blending in the skin tones with anything but oil, or (b) as crisp rendering of the hair and other details with anything except egg tempera (or a tempera grassa that leans toward egg). Which means he was layering mediums, specifically egg over oil, which I understand to be problematic. And yet the altarpiece has lasted well for nearly 600 years. In short (apologies - I'm being anything but short here...) how did Van Eyck do it? Granted, he was one of the greatest painters in history...! Still, I find it mysterious.

(My guess is it is mostly oil; but some of the fine details were done with a relatively oil rich, tempera grassa (so that the paint would adhere well) that was perhaps water based (hence the beading). Just a guess, of course...)

Koo

[Robert Meredith]

Koo: I am oil painter with an academic interest in egg/oil tempera. I believe egg/oil tempera was probably the transition between tempera and oil painting. I occasionally mix up an egg' oil emulsion and use it with dry pigment to add fine detail and highlights to my paintings. It has a crispness that stands out on oil and you can draw a very fine line. It has never given me any trouble.

The beauty of oil is that it dries slowly and can be manipulated on the surface of the painting and produces soft edges. A water based paint dries quickly and stays where you put it resulting in harder edges and outlines. I don't see any real purpose in making oil look like tempera or tempera look like oil but it could be done. To make oil look like tempera I would use oil paint thinned with turpentine to the consistency of watercolor on an absorbent ground like clay board and make tempera like brushstrokes with a fine round sable brush. I would put a drop or two of cobalt dryer in my terps. The turpentine will quickly evaporate and set the paint. If you leave a little space between each if the brushstrokes it will look a lot more like tempera. In a short while you should be able to place additional brushstrokes of transparent color over the previous work being careful not to disturb it. This will dry to a matt surface producing a tempera-like painting. Or, you could just paint with tempera.

[Koo Schadler]

Thanks for your reply, Robert. I have one more question for you. When you mix an egg oil emulsion, what approximately is your ratio of egg and oil? Do you tend toward a water-based emulsion with more egg in it; or is there a greater percentage of oil than egg so that the paint is solvent-based? There are not many people working in tempera grassa and so, when I have the opportunity, I like to ask questions.


Perhaps saying that I want egg tempera to behave like oil is a misleading way to describe my intention. More accurately, I am trying to explore egg tempera's range and see if that range is greater than is generally described. Often I have found this to be true. For example, there are many ways to apply egg tempera that negate the need for hatch stroking; tempera is capable of denser paint and greater opacity in highlights then it is sometimes given credit for; it is possible to apply many, many glazes in order to darken values (although values can't get as dark as in oil); various varnishes can be applied that further increase tempera's saturation. In short, I believe one can get a richer, more "oil-like" look from tempera than is sometimes appreciated.

Which may logically lead one to ask, why not just work in oil...?! As you know, egg tempera has attributes that you can't get in oil: a quick drying time (that allows for dozens of glazes to be applied in a single day), the lack of solvents, the ability to easily render precise and crisp lines. I don't want to give up those, as well as many other wonderful qualities unique to tempera.

I love attributes of both oil painting and egg tempera. If, at any given moment within a painting, I could jump back and forth between the two mediums, to get the best from each, I would - but of course a painting made from a complete jumble of egg tempera and oil wouldn't behave or last. So, I have chosen to work in tempera and try to expand its range.

Conversely, I'm wondering if Van Eyck was able to get an "expanded range" from oil: both beautiful blending and the remarkably fine hairs rendered on Adam's head and body. Could such precise hairs have been rendered exclusively in oil (not in egg tempera, or tempera grassa)? I don't know enough about oil to answer that question..but I'm hoping there's an oil painter out there who can.

Koo

[George O'Hanlon]

Conversely, I'm wondering if Van Eyck was able to get an "expanded range" from oil: both beautiful blending and the remarkably fine hairs rendered on Adam's head and body. Could such precise hairs have been rendered exclusively in oil (not in egg tempera, or tempera grassa)? I don't know enough about oil to answer that question..but I'm hoping there's an oil painter out there who can.

It appears that yes, Van Eyck was able to achieve fine rendering without the use of egg in his paint. While there is evidence of egg tempera in his paintings, it appears that it was used to prepare "lean" colors, that is to say colors with the smallest amount of linseed oil to avoid the issue of yellowing.

[Koo Schadler]

Thanks, George.

The reason this issue confuses me is that, while all that I've been told and my own experience affirm that egg tempera does not sit well a top oil, the binder in egg yolk isegg oil. Granted, the egg oil is emulsified in water (and water based paints don't generally behave on oil), but given that the binder itself is a type of oil (I had one conservator tell me he considers egg tempera a type of oil paint), why doesn't it behave well on other oil mediums? Is it because of the water vehicle? Or because egg oil is not as strong a binder as other drying oils like linseed, walnut, etc...?

To what would you attribute the beading of the paint in some of the fine hairs on Adam's leg?

Also, can you shed any light on why tempera, with its egg oil binder, does not sit well atop other vegetable oil mediums? What are the differences between the two (egg oil vs. drying vegetable oils)?

Apologies if I appear so persistent about this issue...I find few discussion on the transition from tempera to oil, and it is an area of great interest to me.

Koo

[George O'Hanlon]

To what would you attribute the beading of the paint in some of the fine hairs on Adam's leg?

I do not know the direct answer to that question. However, there are numerous ways in which this effect can be done without resorting to emulsions. For example, heat-polymerized drying oils mixed with resins can produce "beaded" applications of paint, simply because these mixtures can develop thixotropic body during application that can maintain a brushstroke on the paint surface.

Observations on painting technique are most often based on a persistent fallacy: that, if some painterly effect or appearance in paint can be duplicated with a particular choice of materials then those materials may well have been the ones used to produce the effect. This type of analysis was applied by Max Doerner in the first half of the twentieth century, for which he proposed that Van Eyck employed a complex mixed technique involving egg tempera thinned with oleoresinous glazes.

To date researchers have found drying oils, heat-polymerized drying oils and pine resins predominating Van Eyck's paintings. In some cases proteins appear but alone and not mixed with drying oils, suggesting that pure egg tempera (not egg-oil tempera) as solitary layers in some passages of his paintings.

Also, can you shed any light on why tempera, with its egg oil binder, does not sit well atop other vegetable oil mediums? What are the differences between the two (egg oil vs. drying vegetable oils)?

Egg yolk is an emulsion of oil-in-water (O/W), hence the oil (the dispersed phase) is dispersed in water (the continuous phase) of the emulsion. The surface tension of water is greater than oil, causing egg yolk to "bead" or puddle on oily surfaces with lower surface tension. The surface tension of water is 71.97 dyne/cm at 25º C.("dyne" is a unit of force—centimeter-gram-second—and is equivalent to the SI units of "mN/m" [millinewton per meter]), whereas that of linseed oil is about 32 dyne/cm at 25º C. This difference in force cause the effect of "beading up," that you observe when painting with egg tempera over an oily surface.