RE: archivalness of gum
<< I think gamboge, lakes, etc. are also names of tones, not pigments, tho
some pigments could be so named, I suppose... >>
I believe many of them are originally pigments of dyes. Like gamboge is a
plant. I have use true gamboge in painting. It is in a form of a stick (they
take the liquid (sap?) from the plant and put in in a thin bamboo trunk and
after it dries, they break the trunk to get the gamboge out). You "grind" it
like you grind an ink stone, but it is not actually grinding, it is more
like dissolving. Gamboge is not permanent, so today it is replaced by more
permanent synthetic version, so gamboge becomes a tone name.
Lakes are all plant dyes and so are very fugitive too. Today's lake are
indeed hue/tone names made from synthetic mixes.
Same with indigo which originally was made from indigo plant. It has a
beautiful purple tone, but it is also fugitive. Today's indigo is a mixture
of prussian blue and quinacridone red. Those who are used to true indigo
sometimes complain that the synthetic indigo is too colorful (the true
indigo is more muted), but this can be fixed easily by adding just a touch
of black; so today's indigo is also a hue name.