egg yolk as an oil paint rheology modifier

Sometimes in old masters' paints, scientific analysis detect a little amount of egg proteins, mostly from egg yolk. This addition is too small to have the paint renamed as a tempera grassa. But why painters did mix this little drop of egg in their oil paints ?
The main reason is to be found in painting practice and paint consistency.
Preindustrial paints were made by artists in their studios, by mulling pigment and oil together. These paints contained no additive. Most of the time, their formula could be summarized as pigment + oil. They were different from our modern oil paints : they were longer and sometimes even stingy, more siccative, less fine and buttery.
Sometimes during the painting process, an artist needed a shorter paint which could be spread in impastos without running or sagging. In such cases egg yolk could become a very efficient rheology modifier as you can see in this little video.

[@T12E5]

Perfect! Using yolk has never been explained for use like this, and solved the issue for how long my paints were coming out after mulling!

[@Lanternamagika]

Amazing indeed! Thank you for showing us so neatly

[@jhb61249]

Nice. Thank you.

[@monte49]

i have tried this - the effects are amazing and very useful for certain effects. As you said, it requires very little to make this happen!

[@thierrymoutard-martin12]

Egg yolk was added in pre-industrial paints. These were made by handgrinding just before use, not intended to be stored. Egg yolk will rotten in a few weeks and the paint will lost its properties. If you want to store your paints in aluminium tubes, I suggest you can add some beeswax as a rheology modifier. It acts almost like egg yolk. A bit less efficient, but still makes short paste.

[@lucianocortopassi3098]

Thank you very much!!

[@lucianocortopassi3098]

Thank you for the tip. can I make my own paint by adding drops of egg yolk and store in aluminum tubes? Is there any problem in doing this? Thank you very much

[@thierrymoutard-martin12]

Egg yolk is a natural animal product. It doesn't stand a long storage in tubes. It is more efficient as a post-addition ingredient. Grind and store your oil paint in tube, then when you take it from the tube add your egg yolk just before using the paint.