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Old 23-07-03, 03:28 PM
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RobM RobM is offline
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Join Date: May 2003
Location: Nottingham, England
Posts: 352
Default Ultramarine Blue

Some years ago I noticed a white residue in areas that only contained ultramarine blue. It was brushed away with a soft clean paint brush. It did return a few more times but now seems to have stabalised. The painting was done in 1996.
I also notice a white crystal type substance on the inside of my ultramarine paste jar. The substance is so light that I can just blow lightly into the jar and it disappears.
In a paper presented to The Society of Painters in Tempera by P. Tudor Hart on July 8th 1922 states...
"This pigment requires most careful manufacture to ensure its stability, its richness and pureness of colour, and as far as possible the elimination of the soluble sodium salts and the excess of sulphur. This latter is rarely, if ever, eliminated."

He goes on with a warning...
"One of the great disadvantages of ultramarine is its characteristic property of being readily acted upon by acids, even by the weak organic acids. I therefore warn Tempera painters against the use of a few drops of vinegar in their egg when tempering ultramarine."

My question....is the white residue salt?

Rob