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  • Metalpoint GroundsApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2019-10-01 15:03:12 ... Most recent comment 2019-10-02 08:11:14
    Grounds / Priming Drawing Materials
    Question

    Hi.  I'm doing more metalpoint ground experiments and have a few questions.

    1. Does anyone know relative hardness (i.e. which is softest, which is hardest) of a cured film of the following binders: gum Arabic, egg yolk, casein, acrylic polymer, vinyl polymer, and oil?    I'm primarily working with water-based metalpoint grounds, but I made an oil ground and found that it works really well; it seems to abrade better than other surfaces, once it has fully cured.  So I'm wondering if an oil base is that much harder and resistant to a metal nib than the above water-based binders. 

    2.  Gordon Hanley is a metalpoint artist who, apparently, (as seen in online reproductions of his work) achieves authentic blacks in his drawings (whereas most metalpoint artists get no more than deep grey).  He says he gets black by working with pure silver on a homemade, proprietary ground. Many metalpoint artists would like to know his secret, but he stays mum.

    I made an oil based ground that consisted of 1 part Gamblin Brilliant White Oil Paint, 1 part silica, 1 part Liquin.  Not sure how durable such a combo is, but it did yield very dark marks - and, after sitting for a few months, I just noticed that one set of marks now appear genuinely black. (To my annoyance, I didn't note what metal nib made those mark).  The questions are: how durable is that combination of ingredients? And is there anything noteworthy in those ingredients that might account for deep grey metal marks turning black?

    3.  I've played with adding different extenders (silica, bone ash, glass, barite, pumice, marble dust, chalk, historic pigments) to metalpoint grounds, to increase abrasion.  Silica gives the best results, which isn't surprising given its Moh's Hardness Scale rating of 7 (harder than any metal nib I use).  What is surprising is that Talc, with a MHS number of 1 (much softer than my metal nibs) also seems to minimally increase abrasion.  I've read up on the properties of talc but it's confusing for a non chemist to distinguish between natural versus milled state, etc. - I just don't understand it all. So my question is, does anyone know properties of talc (i.e. does it have an unusually rough morphology?) that might explain why it improves a metalpoint surface?  Is it accurate that all minerals, even after they've been milled, have an irregular morphology (versus, for example, some modern pigments that are quite smooth and round at the particle level)?

    Thanks as always,  Koo Schadler

Answers and Comments
  • EditDeleteModerator Answer

    ​1. Typically, coatings are not measured using a Mohs scale as are minerals, but there is common test method for coatings, ASTM D3363, Standard Test Method for Film Hardness by Pencil Test, that is often used to determine the hardness of coatings. We have the test equipment for this method, and if you would like to prepare the grounds on rigid panels, then we would be willing to test them according to this test method.

    2. It is difficult to say how durable the paint you created is because we do not know all the ingredients. I could not find a Gamblin Brilliant White in their current catalog. Is it Gamblin Radiant White?

    3. Talc products available in commerce are platy talc, containing predominately the mineral talc; or tremolitic talc, most often a natural blend of talc, tremolite, serpentine and anthophyllite. Platy talcs can be further classified as microcrystalline or macrocrystalline. Microcrystalline varieties are naturally small in plate size and comprise compact, dense ores. Macrocrystalline varieties contain relatively large, higher aspect ratio plates. Platy talcs improve the toughness and general durability of paint films. The talc plates tend to align with the coating's flow so that they are parallel to the substrate in the dry film.

    Talc is a natural pigment ground from natural mineral deposits, hence it can have rough edges. This is also true for many other extender pigments derived from natural mineral deposits, but not all since some extender pigments are made by precipitation.

    The hardness of a coating, such as a ground, depends on numerous factors, such as the pigment type, pigment volume concentration (PVC), binder type, and the type of additives incorporated into the coating.

    George O'Hanlon
    2019-10-01 15:43:35
  • EditDeleteModerator Answer

    George and Koo,

    There are so many time that we are so happy that we are not trying to, stupidly, moderate this forum by ourselves. This was another one of those.

    Brian Baade
    2019-10-01 20:25:47
  • ApproveRejectUn-ApproveUser Comment

    ​Thanks for the replies.  I much appreciate the offer to do a binder hardness test, George - count me in.  And you're correct about the Gamblin, my mistake, I used the Radiant White with safflower oil and titanium.  As you know titanium white is very hard, another excellent pigment for a metalpoint ground.  I'm primarily curious about the black marks generated by my improvised oil ground and need to do more experiments.   

    2019-10-02 08:11:14
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