Storing pigments dry is the most versatile option. Dry pigments can be mixed with any medium aka - oil, egg (for tempera), acrylic and of cause water or "milk of lime" if we are talking about fresco. Oil and acrylic are the easy ones - mixed more than needed - tube the thing! Did not mix enough - mix some more. With fresco, it is somehow different.

<<There has been a big debate
<<on how to store pigments used
<<for frescoes. Some say to keep
<<the pigments dry, others say a
<<paste form is best...

When preparing for painting a fresco every moment counts, more - there seems to be never enough time - I never spent less than 10 hours painting a giornata and I do not do the plaster myself (This is not what one may call a luxurious living but rather a necessity!).

It takes about 4 hours to prepare and lay the intonaco plus about 2 hours to finish the day trim the line and stuff. It takes about 30 min. to an hour to properly grind a small portion of a single base-colour. Base colours: 2 blue, 2 green, 2 red, 2 ochras, 2 umbers, 2 sienas and couple others that I personally like - total 14-15 colour, plus a good portion of "bianco" (not sure about spelling - in English this would be fine ground lime powder ground with water just before painting and at the middle of the day - for white - lives only about six hours. This is the reason to grind it twice a day, after six hours lime "swells up" and it is not good for painting.) Are you counting hours? I got 18 so far. Than add to it couple of hours to prepare the all of the colour tones for the day - mixing then as you go is not very wise - it is curtail to remember what has been mixed the day before, and the day before, and the day before to a "tee" - so all the parts of the body are of the same "race" and no "arlequine (joker)" pants. Now add about an hour to pounce the cartoon and setup - it is good to do the pouncing the same day - helps to tune in and refresh the "lines". So where are we now? Ah... 21 hours and we did not even started to paint yet.

Quite a list (Carter, Gary and Bottaro I am sure you know all that, I just wanted to let the fresco beginners to get an idea), have to get to the point now.

I keep base-colours for fresco ground with distilled water in glass jars about half to a pound of each. Pigment settles at the bottom into a thick paste. Pigments can be kept this way for years. When ready to paint regrind the paste as needed for the day with the "milk of lime". If pigments are ground well from the start then do the mixes and use the time saved for painting.
I also have every pigment in dry form, 2-3 pounds of each.
The only concern with that is the moisture, so far it was OK - no lumps.

Although earth pigments and oxides are relatively inexpensive and widely available, some other kinds are hard to find! For example my special blue - I have half a pound left and clueless what to do when it runs out.
If I would grind it for each fresco separately I will have to throw away what's left unmixed each day. It can be more than half. Bad enough that all mixes have to be chucked!

Ilia Anossov
http://www.truefresco.com