This experiment began in November 2016 and was finished up at the beginning of 2017.
The ripe buckthorn, Rhamnus spp., juice had been extracted from refrigerated, and slightly dessicated buckthorn berries from Saco, Maine. The berries were reconstituted with distilled water (DW), and then rock alum (potassium aluminum sulfate) was added to the juice. Commercially available gum Arabic was then used with distilled water to paint it out on 12/23/16.
By 12/29/16 the dark blue had turned to dark green and further trials were performed.
The change of color from blue to green suggests a few possibilities to me. Either the alum had a chance to change the buckthorn after sitting with it for a while or it could have come into contact with calcium carbonate contaminates from the enviornment like egg or clam shell (neither of which are scarce in my house) or the Strathmore Bristol vellum finish paper is prepared with a buffer that reacted with the acids and berry juice.
Further trials were painted out from the original berry juice with alum in the palette and shell with gum. More distilled water was used to re-hydrate the paint and thin it out for greater visibility.
The juice with alum alone, in a clam shell and with crushed egg shell all yield what I would describe as a gentle sap green. The trial with lye turned from a rather interesting olive to quite a bright yellow reminiscent of the yellow yielded from the green buckthorn drupes and in another ripe drupe recipe discussed here.