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Viewing Topic "Fat over lean and PVC"

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Posted By: George O'Hanlon
Total Posts: 2227
Joined Date: Jun 15, 2004

Ivan, it does not quite work that way, and besides it is not the PVC of the paint that is critical but rather the critical pigment volume concentration (CPVC) that is most important as this is the ratio of pigment to binder that results in the proper amount of binder that envelopes each pigment particle.

George O'Hanlon Technical Director Natural Pigments
Posted : Apr 18, 2017
Posted By: enthusiast
Total Posts: 63
Joined Date: Mar 13, 2015

Adding a paste with high PVC or better stated at the CPVC to paint will not reduce oil content in lower layers, but it will also not change the PVC of paint to which it is added. Both the Velázquez Medium and Underpainting Transparent Base have a PVC of about 55%.

 

I thought it would work like this:

- paint with PVC 40 %, i.e. 40 % pigment, 60 % oil

- chalk paste with PVC 55 %, i.e. 55 % pigment, 45 % oil

If mixed in ratio e.g. 1:1, the resulting mixture would have 47,5 % pigment and 52,5 % oil and therefore it would have lower oil content than just pure paint and thus would produce leaner paint layer.

So, it seems I'm wrong, right? Then, what practice would you suggest to achieve relatively lean lower paint layer?

Posted : Apr 18, 2017
Posted By: George O'Hanlon
Total Posts: 2227
Joined Date: Jun 15, 2004

Adding a paste with high PVC or better stated at the CPVC to paint will not reduce oil content in lower layers, but it will also not significantly change the PVC of paint to which it is added. Both the Velázquez Medium and Underpainting Transparent Base have a PVC of about 55%.

George O'Hanlon Technical Director Natural Pigments
Posted : Apr 18, 2017
Posted By: enthusiast
Total Posts: 63
Joined Date: Mar 13, 2015

The oil absorption of a pigment is determined by adding sufficient oil until a "coherent" paste is formed. This is when the paste does not crack or break but can be smoothed with a palette knife. Whether it looks dry or wet depends upon how well oil wets the pigment and is not a reflection of its oil absorption.


Thank you, I  think I understand what you mean; the paste was indeed smooth. However it seems to me, that this doesn't really answer my question (at least not entirely)... So, I'll try to reformulate my question; assuming, that paste is properly made (i.e. pigment particles are porperly wetted), is adding a paste with high PVC (therefore with low oil content) to a paint good practice to reduce oil content in lower layers?

Now that I think of it; can you tell me, what is approximate PVC of Velazquez medium and Transparent underpainting base?

Posted : Apr 18, 2017
Posted By: George O'Hanlon
Total Posts: 2227
Joined Date: Jun 15, 2004

The oil absorption of a pigment is determined by adding sufficient oil until a "coherent" paste is formed. This is when the paste does not crack or break but can be smoothed with a palette knife. Whether it looks dry or wet depends upon how well oil wets the pigment and is not a reflection of its oil absorption.

George O'Hanlon Technical Director Natural Pigments
Posted : Apr 4, 2017
Posted By: enthusiast
Total Posts: 63
Joined Date: Mar 13, 2015

There is a remark about fat over lean rule in the thread about alkyd resin: this principle inherently flawed and is better replaced by understanding the pigment volume concentration or PVC of the paint.

I bought fine grade chalk (item 473-11). I'm not sure whether to use specific gravity or one of densities in calculation of CPVC. Specific gravity gives me lowest CPVC, but it is still 80,29 % (which is very high I guess). I mulled some chalk with linseed oil. Of course I don't know what PVC I achieved; I had to add a bit more oil, because the paste was rather stiff when I mulled it first time. But let's assume that the result could be still rather lean.

According to this article: http://www.justpaint.org/volume-weight-and-pigment-to-oil-ratios/ raw sienna has CPVC 45 % and raw umber 34 %. I guess it can vary a bit as there are many grades of sienna and umber. To achieve lower oil content in the first layer, I decided to try mixing one part of raw sienna/raw umber mixture with one part on the chalk paste I made.

Resulting paste was thicker than pure sienna/umber mixture, it had short rheology and itwas not very easy to spread. I had to add a drop or two of spike oil. That worked better, but the paste still looked sort of "dry"; not sure how to describe it. The painting was done on semi-absorbent acrylic "Gesso" by Lascaux. Seeing how „dry“ the paint feels I was worried at first that this practice could result in underbound paint layer. The dry layer is matte and it feels like it has a slight tooth when moving across the surface with my finger (the pigments were slightly coarse). However, even when I use some pressure, my finger is clean, i.e. there isn'tany pigment dust coming off the layer.

It seems to me, that the paint film is indeed coherent and the practice I choose is correct. Am I right?

Posted : Apr 4, 2017

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