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MITRA Forum Question Details

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  • Mixing chalk directly into your paint while painting.ApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2016-11-29 14:10:40 ... Most recent comment 2016-11-29 14:17:00
    Chalk Oil Paint Other Paint Additives Paint Mediums
    What are your thoughts regarding mixing Chalk, calcite, barite, kaolin (clay), talc, silica (quartz) and bentonite directly into the paint or into the medium while painting. I love some of the effects that are possible when you add chalk or barite into your paint on the palette, but I'm worried about permanence. I don't use any mediums except for linseed oil and or stand oil.

Answers and Comments

  • EditDeleteModerator AnswerNone of the additives you have listed have any issues relating to "permanence." We find some of these additives all the time when we analyze historic paint samples and they are commonly found in modern paints today. As with any paints you make yourself the greatest concern here is not to over-step your bounds when it comes to the pigment (and filler)/binder ratio. Too much pigment and/or filler can result in an underbound paint film (think powdery and friable). Companies that manufacture paints have conducted dozens of tests to find suitable ratios that will in turn create stable paints. You can try to trouble shoot this on your own by adding various amounts of fillers to specific tubed colors...also be sure to disperse the fillers as well as possible as you do not want to end up with minute "pockets" of dry material throughout your paint film as these can also lead to a weaker paint film. In sum, there is nothing wrong with adding such materials. But it is possible to add too much.
    Kristin deGhetaldi
    2016-11-29 14:25:36
  • EditDeleteModerator AnswerAs Kristin pointed out, the permanence of these dry fillers is not the problem here. The issue is one of creating a paint film where the pigments and modifiers are well dispersed and with no pockets or agglomerations. The other essential aim is to create a paint that contains enough oil to properly bind for the layer you are working on. There is really no way to control these important considerations following the practice you describe. It would be far preferable for you to completely disperse your modifiers into one of the oils that you use. You could make these as lean or as fat as you wish, as long as they are well dispersed, and then judiciously add them to your raw paint. You will have far more control and likely create a better paint film.
    Baade, Brian
    2016-11-29 15:36:17
  • ApproveRejectUn-ApproveUser CommentI agree, while it might not feel as improvisational to batch-mix paints as to work entirely on the palette, combining fillers with colors on a slab is the better approach. The modified paint can then be tubed for later use (this procedure is easy to learn and not nearly as widely used as it should be.) It's possible to crudely evaluate flexibility and adhesive power of filler-modified paint by applying samples to heavy mylar sheets. Once the sample has dried, the mylar can be flexed to see if the film splits or detaches easily. Another advantage to batch-mixing is reducing exposure to powders, since airborne particles can pose a health risk when using some of the fillers mentioned. Even some calcium carbonates can contain asbestiform particles- a few years ago, a popular brand of marbledust was recalled until this issue was corrected. Matthew Kinsey, Utrecht Art Supplies
    2016-11-30 10:25:52
  • ApproveRejectUn-ApproveUser CommentWow! Thank you. that's a great answer. I guess, I'll skip the improvisation and batch mix a few colors this way. Maybe make my own Velasquez white.
    2016-12-05 20:09:02
  • ApproveRejectUn-ApproveUser CommentIt was good enough for Rembrandt and especially Velazquez. Also refer to the Rembrandt research project. Many old masters added calcite to their paint ; even powdered glass was sometimes used.
    2017-01-28 23:20:01
  • EditDeleteModerator Answer

    ​Please see my response about calcite and other subjects on this thread.

    Yes, powdered glass has been found in the works of some of the Old Masters. There is still debate as to whether this was for rheological reasons, adding translucency, as an additive to promote drying, or possible a mixture of the above.

    Baade, Brian
    2017-01-29 14:10:58
  • ApproveRejectUn-ApproveUser CommentCalcite is a common constituent of sedimentary rocks, such as limestone. Calcite is also the primary mineral in metamorphic marble. Chalk is a form of limestone. Hence, marble and chalk are all forms of the mineral calcite. There are also human-made forms of calcium carbonate, known in the industry as precipitated calcium carbonate or PCC. Calcite, barite, kaolin, talc, silica and bentonite are all white pigments of low refractive index (less than 2). They are also known as extender or filler pigmentsin the paint industry, because they do not add to the opacity of paint, but can be used to increase the bulk of paint without lowering the pigment volume concentration (PVC). They also add important properties to paint, such as hardness, abrasion resistance, decreasing bulk weight, etc. Extender pigments are effective in bulking artists’ paint and adjusting the opacity, and a much more effective means of decreasing opacity by adding medium or oil to paste paint, which lowers the PVC. However, I must also caution about the excessive use of extender pigments in paint, because if used excessively they will result in more pronounced yellowing of the paint film. The best way to use extender pigments without increasing the PVC of the paint and the subsequent danger of creating an underbound paint film, is to grind the extender pigment in oil to form a stiff paste prior to adding to paint. —George O’Hanlon, Natural Pigments
    2017-01-29 16:05:54

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