GReetings,
When making oil paint or modifiying oil paint with additives, how do you determine the ratio of pigment to oil, say titanium white with marble powder to linseed oil?
There is a point when the paint becomes very thick and will even roll off of the mixing plate glass, and this is obviously too much pigment to oil. Are oil absorbtion rates needed, if so are these online?
Thank You
Excellent, thank you for the comments and Golden link.
When grinding your own colors, paste consistency on the slab is not always a good guide to make your final paint as intended. I work with a natural green earth which gain consistency; the more I grind the paste, the stiffer it becomes, and more viscous. Real viridian (the one we call “vert émeraude” here in France) behaves exactly at the opposite : I make a stiff paste, and when I grind under the muller, it becomes more fluid and runny.
Another issue in OA tables published in the literature is the normal procedure for the test. Usually it is the Gardner Coleman spatula rub-out. With this method, the paste is not ground. It only undergoes a few minutes working with the spatula, which can give a very different OA figure.
Are you asking in reference to small-batch, handmade oil paints? If that is the case, consistency on the slab is a very good indicator of optimal proportion. Just watch the product develop, and gradually add oil to the pigment until a nice, workable paste is achieved. Regarding published ratios, Golden included average absorbency for common colors in a great “Just Paint” article last year: http://www.justpaint.org/volume-weight-and-pigment-to-oil-ratios/
As Matthew said, the feel on the slab is ultimately the best guide and part of the art of grinding your own paint is gaining experience through trial and error. I would caution against using oil absorption rates that you can find published. They invariably come as ranges which will not tell you about your specific pigment. And to be honest, how even oil absorption rates are calculated is weighing how much oil you need to add to 100 grams of pigment until it is a stiff spreadable paste. So, your right back to that sense of feel. And we would encourage you as well to read the article Matthew points to as it should put some of these issues in context and also point to a common misconception between oil absorption rates in terms of weight versus volume. Lastly, understand that paints can feel different after they are allowed to rest for a period after mulling as the oil will continue to wet out the pigment and the paint can sometimes stiffen or loosen up as a result.
Anyway, making your own paint is a great way to learn and explore the nature of pigments. Keep accurate notes and if you can get a accurate digital scale it will be easier to know how you can repeat something that you like.
Sarah Sands
Senior Technical Specialist
Golden Artist Colors
Sarah makes a very important point about variations in pigment that can’t be predicted by “recipe” alone. Even in paint manufacture where a proprietary formula is used, each batch of pigment is tested by hand by making what is essentially a small amount of paint.
In addition to Sarah’s advice to examine the paint body after it rests, I would add that it’s not always good to try making the stiffest paste possible. The paint vehicle has an important optical function in bringing the color to its best advantage, something that only works when clusters are reduced and each particle is enveloped in oil. Some pigments can be used in fairly high proportion to vehicle (Dutch Boy White Lead was 88-89% pigment) but a dense putty that releases from the slab is not only underbound, it also makes movement of the muller impossible.
I had planned to add my own experience and thoughts on this but you two have it well covered. Thanks
