Zinc White (PW4) is a cool, semi-transparent white pigment composed of zinc oxide which has been used in artist paints since the 18th century. It is commonly found in paints labeled as Mixing White and Transparent White, and is also often added to Titanium White (PW6) to improve its handling qualities. Recently there has been much discussion about Zinc White’s suitability for use as a pigment in oil paint, with concerns that zinc oxide causes a brittle paint film that can crack and delaminate.
In this article, we explain the pros and cons of Zinc White and answer some frequently asked questions about its use in our own Professional and Artist oil paint ranges, including best practice when painting with it.
Text provided by Paul, a member of our technical team
It certainly appears that the Zinc White question is heating up; with lead duly buried and cadmium put at ease, it looks like there’s a new bad guy in town.
It’s fair to say the issue is not a new one. Zinc White has been used in artist paints for 250 years and is still in wide use today. As with many pigments, it comes with its own characteristics — some good and some bad.
What are the concerns about Zinc White?
The concerns around Zinc White were well documented, even back in the early days of its use in oil paint. It’s clear that zinc oxide reduces the flexibility of paint, and less flexibility means a higher risk of cracking when stressed. This brittleness develops over time and is not usually obvious within the early months of a painting’s life. It starts to become apparent thereafter when compared to non-zinc oxide pigments. On this point, you should keep in mind that linseed oil also continues to lose flexibility as it ages, with any given painting having more flexibility at 50 years than it will at 150 years. Unfortunately, the reduced flexibility seen with Zinc White is not yet fully understood and continues to be investigated.
Paints containing Zinc White do not consistently create a problem. In most cases, the degree of flexibility is still sufficient for a well-supported painting. After all, Zinc White has been well utilised in mixing whites for a long time so we may assume that a large majority of paintings out there will contain some amount, yet we don’t see wide-scale cracking or delamination.
That leaves us with trying to understand if we have a problem or not, and what real advice we can give.
What are the advantages of Zinc White?
- It offers a very clean, cool white compared to other white pigments.
- It offers a more transparent white, that makes it more gentle and subtle in mixes.
- It reduces the yellowing and improves handling of other white pigments, including Titianium White. It is, in fact, often added to Titanium White oil paint for this reason.
- It improves lightfastness.
- It has a long open time.
- It suppresses mildew formation on the paint film.
- It is unique. There are no other cool white pigments with the same properties as Zinc White.
Below is a comparison using paints in our Professional and Artist oil range. Small amounts of Titanium White and Zinc White were added to Alizarin Crimson, Burnt Sienna and Ultramarine Blue to see their comparative effects in mixtures:
Titanium White
Titanium White has a very high tinting strength, and just a small amount increases the opacity of the colour and gives it a pastel effect. Many artists find it’s covering power useful, but it can be overwhelming as a mixing white.
Zinc White
Zinc White is more subtle than Titanium White. More is needed to have a significant effect on the colour, and it tints the mixture without adding opacity or chalkiness.
Do Jackson’s oil paints contain Zinc White?
Some do. The pigments used are stated on the label of all of our paints. Zinc White can be identified by the pigment index number PW4.
How much Zinc White do they contain?
This varies for each colour. Our paints each have their own uniquely balanced formulas, the ingredients of which may each play a part in its flexibility. Pigment to oil ratio, oil types, co-pigments and enabling ingredients will all play a part.
Why do you include Zinc White in your Titanium White oil paint?
Used alone, titanium dioxide (known as Titanium White, pigment index number PW6) makes a long, stringy paint which remains relatively soft when dry. Adding Zinc White improves these qualities and, as stated above, improves lightfastness and non-yellowing properties.
Do these concerns affect Zinc White watercolour and acrylic paints?
No, there is no evidence that Zinc White causes cracking or delamination in acrylic or watercolour.
Will oil paints which contain Zinc White crack or delaminate?
Oil paints containing Zinc White will be more brittle than those that don’t. However, there are many variables involved including what the paint is mixed with, how it is applied, how it dries, and what it’s exposed to during its life.
We formulate our Artist and Professional oil paints to be sympathetic towards flexibility, and we have taken a look at older samples of our zinc-containing paints. Whilst less flexible, we cannot see any dramatic failures in the aged dry film. We therefore feel that the formulation aspects present a sound basis for zinc white paint and, provided the paint is used with good practice, should not produce any significant concerns.
What is best practice when using Zinc White in oil painting?
- Avoid use in underpaintings or lower layers.
- Do not apply thick, heavy layers. keep as thin a layer as needed.
- Avoid adding white spirits or solvents.
- Use of additional oil, especially linseed stand oil, is believed to be beneficial.
- Use sturdy supports and avoid stretching, bending, or flexing.
- Minimise transportation or shipping of the finished painting, package well with supportive transport frames and supports. Label clearly as ‘fragile’.
- Keep the painting in a moderate environment as possible, ideally 16-25°C and 40-60% relative humidity. Avoid heavy/sudden swings in temperature.
The last three points are good practice for any oil painting, not just those which use Zinc White.
Oil paints on jacksonsart.com:
19 Comments
I guess some key questions concern the difference in the pigments
used. We know the chemistry of both but how much do we know
about the size of the pigments used, their method of production,
pigment particle size and so on. So both are used in sun block as
uv absorbers but they do have different properties and different
efficacy with respect to there interaction with light. There is a fair
amount of knowledge there. And I guess we would need to decide
systematically what the difference between the two films were. If it
makes no difference in acrylics(?) it suggests zinc oxide is
impacting linseed oil and either the rate of reaction and/or the type
of reaction. I guess I might add that if my painting is sitting in a
gallery in 100 years time and people complain about a bit of
crazing I would be happy that they’re still looking at it!
I make my own linseedoil paints for my
own use, i refine the oil and boil it to
further remove mucilage and speed up
the drying. If you do not add zinc white
mildew will come on horizontal surfaces
very quickly.
I add some eucalytus oil, this seems to
help against mildew too.
Great advice about the eucalyptus oil, thank you for that
Worth reading
Michael
Firstly, thanks for posting this, it’s an important topic for sure and one to keep an eye on in the coming years.
I hadn’t really taken much notice of this topic until GOLDEN/Williamsburg published their findings back in early 2018. While they don’t replicate real-world conditions, the results are at least indicative and quite alarming. The company have since reduced ZnO content across their range and now offer a titanium white with no PW4. Additionally, Michael Harding eliminated PW4 from their range only this year.
I hope you’re right and there’s nothing to worry about, but I’ve started to err on the side of caution and phase PW4 out of my pallet, particularly on larger canvasses. I’d just like to put it out there that I would be very interested in a Jackson’s-brand white that is free of Zinc Oxide!
Thank you for your comment, Kevin! We’ll certainly keep an eye on the research
Looking at that comparative mixing chart
makes it very clear that zinc white and
titanium white perform completely different
roles. Thank you for that.
Thank you, Frances. I agree they each have completely different roles to play in a palette
Thank you Jackson’s,
Please continue with your advice. Let not
our knowledge be lost.
Thank you for your kind comment, Philip
I would not believe that zinc white PW4 increases lightfastness – handprint.com (which is basically the holy book of pigment info, though only from watercolor perspective) states that PW4 does actually reduce lightfastness in mixes.
Plus, there’s already enough of tests to show how quickly mixtures with PW4 become brittle, irrespective of amount of PW4 added. Plus enough of talks about soap formations, which affects even paintings on rigid surfaces (though this area is still quite blurry).
Yes I think it’s misleading to say zinc
white increases lightfastness. Probably
something was lost in a rewrite. It should
read something along the lines of: “Zinc
white decreases lightfastness of
vulnerable colours in tints less than
Titanium or lead whites.” I don’t
understand or have looked into the
mechanism of this effect with the
different whites, but it has been often
reported.
Hmmm. I must say that I wish you WOULD discontinue the use of
PW4 – I agree with you that cracking and delamination are not
inevitable if it’s used, and you’re right that it’s been around for
quite a while. But we can still get lead white (not from you,
unfortunately!) and I much prefer to use that: the research
evidence is impressive, if not finally conclusive – and it leads me
well away from Zinc Oxide in oil paint.
Thanks for your comment, Robert! It’s up to artists to make the final decision, of course, and we will keep an eye on the research as it develops.
Hi Evie, thanks for this blog. I recently
visited my regular art shop to find zinc white
‘being phased out’. A local brand Langridge
has introduced a new white to their range:
Tinting white, which seems to use a mixture
of Titanium white (PW6) and Barium
sulphate (PW21). What do we know about
Barium Sulphate and what makes it a
reliable/trustworthy alternate to zinc? What
advantages/disadvantages are there with
PW21?
I’d love to know as I often work on canvas.
Hi Lars, thanks for such an interesting question. PW21 usually has very good lightfastness and is a stable/inert material, plus it’s been used extensively as a filler in artist paints for a long time. It’s important for the tint strength of a mixing white to be moderate, so it’s likely the percentage of PW21 is very high in a PW6/ PW21 mixture (Titanium dioxide is very opaque). One issue might be that PW21 would do nothing to counter the issue of Titanium White’s sponginess in oil paint (Zinc White does a great job at offsetting this), so other additives will need to be used. Another concern is that PW21 is very transparent in oil– far more so than Zinc Oxide. The high transparency of PW21 is because of its low refractive index and, as the linseed/safflower oil ages, its own refractive index will increase meaning that the paint is likely to become more and more transparent. Pencil lines and previous layers of paint may slowly reveal themselves over the years. Also, depending on the amount used, there may be a high susceptibility to yellowing, this is because the higher transparency of PW21 will allow the yellowing of the oil to be more pronounced. Research into artist pigments continues and conclusions change all the time! So we’ll continue to watch out for new developments.
Hi Evie,
Looks like the problem is due to formation of zinc soaps: “Moreover, the occurrence of zinc soaps in oil paintings leading to defects is disturbingly common, but the chemical reactions and migration mechanisms leading to large zinc soap aggregates or zones remain poorly understood.”
https://www.cambridge.org/core/journals/microscopy-and-microanalysis/article/electron-microscopy-imaging-of-zinc-soaps-nucleation-in-oil-paint/B442AD4847D1ABF091A4A43CA4C0E2A2#
More info here:
https://pubs.acs.org/doi/abs/10.1021/acs.analchem.9b02443#
These papers raise the concern that eveen mixing zinc with other whites (Lead, Titanium) can still lead to problems. Although clearly these papers don’t describe the whole story.
Thank you for the extra information, Richard
I did a number of oil paint medium tests
with zinc white some years ago and found
the only one to increase flexibility was
linseed stand oil. Which I see you also
recommend. It’s not something that I fully
trust however, or at least, while I can still
purchase and use lead white, I won’t and
don’t use zinc white.
It’s possible that the zinc white
embrittlement has been so omnipresent in
oil paint for so long that I think most
people just accept the effects as “just
what oil paint does.”
So while I think it should be taken
seriously, it shouldn’t IMHO cause
reactions, alike to that of Edvard Munch’s
Scream.