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Thank you for your replies. I like to mix white with 2/3 Ivory Black and 1/3 Umber. The umber accelerates the slow drying of the Ivory and the titanium and at the same time gives a pleasant slighly warm gray.
Be careful! Flake White is white lead which can be highly toxic if you are making it on your own!!
Someone mentioned that they use a gray outdoor, acrylic latex paint to stain their canvases. It drys overnight. I've been thinking of trying that.