Reactive and Non-Reactive Pigments

[George O'Hanlon]

The reaction of natural and synthetic mineral pigments with binders containing lipids, such as egg yolk, drying oils, etc., has been reported by various authors.

Generally, the formation of metallic soaps with the free fatty acids in the binders is assumed to be the reaction. These metallic soaps may act as driers, accelerating the oxidation and polymerization of the binder and providing a hard and resistant pigmented film.

Usually, and without much verification, pigments such as lead white, umber and zinc oxide, etc., are said to accelerate the drying of binders. The synthesis and use of metal soaps, such as those from lead, cobalt, manganese, are common in the paint industry and are used as driers in artists' colors.

A study was initiated a few years back that produced a definitive answer as to the formation of metal soaps by natural and synthetic mineral pigments. The researchers studied the formation of metallic soaps of pigmented paint films when mixed with linseed oil.

Non-Reactive Pigments
Lead sulfate
Chrome yellow
Chrome orange
Red ocher
Yellow ocher
Sienna
Umber
Manganese brown
Manganese blue
Manganese black
Cadmium yellow
Zinc yellow
Azurite
Copper blue
Malachite
Cobalt blue

These pigments did not form metallic soaps, so it is reasonable to deduce from this that these pigments do not react with linseed oil. Interestingly, some of the pigments appearing in the group above have long been designated as ones that are reactive, forming metallic soaps with linseed oil.


Reactive Pigments
Minium (red lead)
Massicot (litharge)
Cobalt black oxide
Casell yellow
Lead white
Verdigris
Naples yellow
Cerulean blue
Zinc white
Cobalt blue-green
Cobalt green
Bismuth white

These pigments are arranged in order of their reactivity (from the most to the least) in their formation of metallic soaps.

[GadFly]

I have noticed that a Cypress Umber mixture made from natural pigments with Alizarin Crimson (because of short supply I was forced to use industry grade tube paint that I assumed was synthetic) and refined linseed (Gamblin), had nearly crystalized on a very large painting that I was working. This occured almost as the paint was hardening. Over time and subsequent overlayers of glazes(both scumbled and transparent)this part of the painting became particularly rich and exhibited a luminosity that was remarkable, a beautifully ghostly shadow. However, this particular passage of the painting was jumping off of the linen and could be characterized as a "polymerizedd film". Fortunatly when I varnished the paint a year later it came together in unity. I have to say that I left the umber particles rather large in the grinding process and the paint itself was particularly short. I wonder what happened, but I was able to effectively utilize this "problem". Thank You.

[Robert Garrett]

George,
We had a conversation about reactive and non reactive pigments awhile back, I just wanted to touch on that again.

If I understood correctly the pale grinders oil will work better with the reactive pigments? Also from that conversation I inferred that vermillion was a non reactive pigment. How do I go about finding out if a pigment that isnt on this list is reactive or not?

RG

[gnomo]

George,
Could you expand on the reactivity of Verdigris? I have read conflicting and confusing info. on the use/reactivity of this pigment. Does it need isolation? Can it be intermixed with yellow ocher, for example to make a yellow green, or does this need to be underpainted in y. ocher first, and then glazed w/ Verdigris?

Also, I have heard from some painters that their Azurite in oil had started to turn greenish after not even 5 years. I have read some allusions to this in the Merrifield (I would need to dig this out to site the particular MS). Please explain, and, have you had problems in tubing this pigment?

[George O'Hanlon]

Pale Grinders Oil is best used with reactive pigments and pigments that are difficult to wet, such as organic pigments, because it has a high free fatty acid content.

Vermilion and cinnabar are considered non-reactive pigments, mainly due to the fact that it is a sulfide, and most sulfides are non-reactive to fatty acids drying oils.

There are no published lists of reactive versus non-reactive pigments that I am aware of, except the one cited in the article.

[George O'Hanlon]

Verdigris is neutral copper acetate and failry reactive with fatty acids in drying oil. It has been advised in literature to isolate it in paint layers and possibly with a varnish mainly to protect it from moisture, since acetates are highly soluble in water, and this appears to be the main cause of its alteration.

Azurite is liable to convert to a form of malachite, but the main problem with it is its low refractive index and that with the darkening of oil it can cause it to appear greenish with time. There is no problem making the color, but it too needs to be protected under varnish.

[llawrence]

I have read that verdigris is incompatible with lead white. Do you know if that is true? And what about the other copper pigments, green and blue bice and verditer? Can they be mixed with lead white with no problem?

[Saturn]

Originally posted by llawrence:
I have read that verdigris is incompatible with lead white. Do you know if that is true? And what about the other copper pigments, green and blue bice and verditer? Can they be mixed with lead white with no problem?

Verdigris is not meant to be mixed with white. It was used most of the times pure or mixed with yellow lakes and added as glaze mixed with either heat-bodied oil , resin-oil varnish or just essential oil varnish, which function to effectively "lock up " the pigment.
Used with pure oil it is not resistent to water as George told you. This is why Leonardo directs that if verdigris was ground in ordinary simple oil it has to be varnished immediately after drying. It is better used dissolved in essential oil-resin varnish.
From what I know the most stable and nonreactive of all copper green pigments is the natural form of basic cooper carbonate- Malachite.
Providing that this is the natural stone product.
The artificial forms are not as permanent as the natural ones.
Your question about the reactivity of the pigments cannot be answered with one sentence only - can it be mixed with other pigments or not. Because the change if happens depends on many factors. First is the purity of the pigments.
Cadmium if not pure and contains free sulphur can also produce blackening when mixed with white lead. This does not mean that the white lead should not be mixed with cadmium paints, but that the white lead and the cadmium paints you are using should be pure. then no darkening, no other problems.
I have used natural malachite ground up to paint by me with various binders. In tempera this pigment is very transparent, but when applied on several thin coats one above another it produces excellent, opaque and intense colour which in such a form has tood more than 1400 years unchanged. In oil this pigment is quite transparent too. I mixed such natural malachite oil paint with cadmium pigments just in order to see if any reaction will occur. After one year I did not see any change nor darkening of the paint.
The verdigris should not be mixed with any other pigment, but isolated with varnish.In such way it preserves perfectly.
I would also not mix green or blue bice with any white lead.
The natural Azurite and Malachite as I said can be mixed with white lead without problem because they are not so reactive. At least this is what many old masters did with great success.
Azurite is so stable and non-reactive, that we find it in the paint samples from the XVI and XVII centuries mixed with lead-tin yellow for producing green paint, which could be lightened with white, in contrast to any Verdigris , which should be used pure.
This use of the pigment pure and isolated is more recommended for Verdigris as it is the most violently reactive of all cooper pigments, together with the cooper arsenate( Paolo Veronese green and the various Shelee, etc.) The green and blue bices are not so reactive like Verdigris, but
it is better not to mix them with white in an oil medium.
Not long ago I saw more than 10 paintings by Frans Post on wood, which had all the folliage parts that were originally green destroyed and turned deep brown- black like opaque coat of asphalt because of the incorrect use of Verdigris( it was not properly protected).

The durability of a mix of pigments also depends on the binder in which they are ground, the levels of humidity, the temperature, the presence or absence of direct and difused natural ultraviolet light, etc.
For fast investigation if the pigment mixture changes you can use a simple mix of wto or more pigments in pure distiled water used as a "binder"
. Separate the mix in two parts.Use the first part for refference and put aside for later.
take the second part of the mixture and put it in a porcelan glass and heat it for some time. The heat, the humidity and the presence of air greatly accellerates the chemical reactions between reactive pigments. If after time the mixture is unchanged you can conclude that if these mixtures did not change in a mix without binder, they will even less in a binder of oils.

[llawrence]

Saturn, thank you for your detailed reply.

Originally posted by Saturn:
From what I know the most stable and nonreactive of all copper green pigments is the natural form of basic cooper carbonate- Malachite.
Providing that this is the natural stone product.
The artificial forms are not as permanent as the natural ones.

...

I would also not mix green or blue bice with any white lead.

Do you know what it is about the synthetic versions that make them less permanent, and less wise to mix them with lead white? They are chemically the same as the natural versions I think, only different in particle size and perhaps some impurities.

[Saturn]

Originally posted by llawrence:
Saturn, thank you for your detailed reply.
Do you know what it is about the synthetic versions that make them less permanent, and less wise to mix them with lead white? They are chemically the same as the natural versions I think, only different in particle size and perhaps some impurities.

The natural and the artificial pigments are chemically similar, but are not the same.

The natural malachite has monoclinic-prismatic cristal structure, while the artificial forms have spherulitic crystals, by which they are beying distinguished when analized.
The cristal structure of a pigment is very important because it not only has an impact on the " beauty shape" of the pigment, but also on it's phisical qualities. Perhaps good example will be the Mercuric Sulphide( the famous Vermilion). When it changes it's chemical structure- from amorph to cristal and the reverse it also changes it's colour. When producing artificial vermilion for example first the product is black, then it changes it's structure and becomes red. When the vermilion blackens on exposure to light it does not change its chemical formula, but changes only it's structure, and hence it's colour.

The natural and the artificial cooper blue pigments are not exactly the same. For example many natural blue Azurites are Hydroxo copper carbonates, while the blue bices are Basic Copper Calcium Carbonates. The calcium comes from the action of lime on the unfinished green product.
The things I mentioned above do not explain all the behaviour of the natural and artificial pigments, but perhaps this could give you some direction into which you can start thinking.
Same way it is said that the natural Zinober is more stable than the artificial. If you see the behavior of the natural malachites in paint and compare them with the artificial ones you will understand what I mean . One does not need to be chemist to see the changes of these pigments observable in the old paintings.