MITRA Forum Question Details

Hello there,

I won't have the definitive answer, but some skeptical thoughts, if hopefully they can feed your research.

I'm not familiar with this saying about lead (pigment I assume) binding efficiently over copper. As far as anti-corrosion treatments go for metals, I'm more familiar with zinc based pigments over steel. But lead carbonate directly over metallic copper, that sounds risky.

Lead carbonate has been apreciated for the flexibility it gives to linseed oil films and its drying property, but it's also been historically problematic for its chemical reactivity, turning darker into lead oxide notably. What I recall most was to avoid mixing lead and cadmium colours because they react with each other, but in the end it is a quite reactive pigment. I believe Zinc white is less reactive usually and titanium white even more inhert.

It's likely there is an unfortunate electrochemical reaction between your metallic substrate and the lead pigment, which stains badly your paint as you consume the pigment and turn it to darker oxide (and likely carbonate salts). It's not a good surface for long term stability. Like painting could change colour over the years, you risk your painting bubbling and delaminating over this dust. Furthermore since the grounds are pretty lean in essence.

I don't recall enough about the electric potential of the lead, zinc, titanium and copper oxides against each other to forecast them accurately, but furthermore if you're painting in alkyd which will keep some inherent flexibility (over the solid substrate anyway), it's worth testing the two other types of white.

Alkyds have pretty good adhesion, drying and flexibility anyway, so I'm not sure what are the supposed benefits of focusing on lead white anymore. I'd really advocate forgetting the old lead. 

If zinc and titanium also degrade rapidly (which is far less likely than lead. And I really doubt you can change the oxidation of titanium white like that.), then what you need is to make a... ground before your white ground. A thin layer of your alkyd or resin could seal the surface and insulate the following coat and pigments from reacting directly with each other.​

Good luck,

Lussh

Hello Lussh,

Thank you so much for such a quick and thorough reply!

That’s really interesting, aha I wondered if it was a lead oxide I was seeing when it turned grey/black! Good to know about not mixing cadmium pigments and lead too.

In my case I’m just using lead carbonate in the ground, then painting with titanium white, so hopefully any darkening wouldn’t be visible down the line (I’d heard that titanium white was more likely to darken than lead white, that’s interesting!)

But if there’s any chance it’ll risk my painting, I won’t use it – previously I’ve used Gamblin Oil Ground (with an alkyd binder) which seems to bind very well to metals, but I thought a lead alkyd ground would have been an upgrade! Since it’s touted as being designed for copper and ACM.

I wonder, is this a reaction that would happen anyway at the interface of the metal and primer, or have I created it somehow by brushing more vigorously do you think?

Thank you so much for your time!

Ellie

Hello Ellie,

Alkyds themselves bring the main advantages of lead white (in linseed oil) without the inconvenients. It was arguably a necessary evil in linseed oil colours, far less in alkyds. You'll be safer with titanium white definitely.

If my hypothesis is correct, brushing vigorously caused enough interactions of over-thinly coated pigments to react electrochemically with the copper and oxygen. 

My worst case scenario would be the pigment closest to copper over time eventually darkenning, causing delamination, dark spots or blisters appearing. But it's unlikely to propagate much (like flash rust on steel would) if you don't move it around and keep enough binder between the pigments and metal. (making an isolating ground layer would protect from the effect I suspect).

Good painting to you,

Lussh