For starters, think of a brown paper bag...
Imprimatura is a term used in painting. It is an initial stain of color painted on a ground (like my gessoed canvas shown below).
It provides a painter with a transparent toned ground. I prefer to use the approximate color and value of a brown paper bag.
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The term is Italian and literally means ¨first paint layer" and it helps the classical painter begin with a middle tone and then establish value relations from dark to light.
I cannot think of anything more impossible to paint on than a stark white canvas!
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An imprimatura is usually made with an earth color - for a portrait I like to use Raw Umber mixed with Winsor-Newton's Liquin Medium.
Some people use turpentine + color but I prefer to seal the canvas with Liquin.
Note the cheap "hardware store" white britstle brush above...I "use 'em and then I lose 'em.".
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The first layers of a painting establish value and composition, color comes much much later.
Here are some examples from the works of Rubens:
In his oil sketches, you can see the imprimatura underneath it all.
It is easy to see - especially in these works.
I once saw a show of his oil sketches at the Museum of Fine Arts in Boston and that is when I first understood the importance of the imprimatura.
The imprimatura is the "mother color" that makes it all hang together. The painting above appears to have a darker imprimature - and thus is a darker painting.
This painting bleow - I have not see the original - but it is a mini-lesson in how to (sometimes) underpaint paint dark areas and shadows.
All of the darks and shadow areas appear to have been painted in red - a clever way to make those areas luminous, warm and lively when overpainted.
Note that in a landscape, I'll often use an "earthy red" (or sometimes cadmium orange) as my imprimatura and allow that color to peek through the final layers of the painting.
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Even in the Old Master's drawing, a toned paper is used....and it is just the color of a brown paper bag!
Many of my paintings at KarinWells.com begin with an imprimatura.