Storage of Plant Dyes
STORAGE OF PLANT DYES
It is most important to avoid any exposure of stored plant dyes to moisture so be sure you wrap the package airtight.
Henna will retain its dye strength several years and perhaps longer, if frozen in airtight containers. It can also be refrigerated or kept in a relatively cool dark place at controlled room temperature. Some of us have kept henna for hair use (as opposed to body art use) for well over a year at room temperature with no appreciable loss of dye strength.
Cassia can be kept in the freezer, fridge or at room temperature. It will keep well for a year at least.
Indigo and buxus should be stored at room temperature, not frozen (brief freezing in transit is usually not a problem it seems, but for storage it is not advisable to freeze it). It is best to plan on using your indigo within a year, as it loses some dye strength as it ages and that happens faster with indigo than the other plant dyes. Presumably buxus would be similar to indigo in this regard.
Henna paste will last well for up to 6 months in the freezer. It's best to let it start to release dye (so it's staining your palm light orange at least, within a couple minutes) before you put it in the freezer. Freezing makes the dye a little stronger than usual. It can be briefly thawed and then re-frozen several times before it loses much strength. Freezing puts the dye in “suspended animation,” so to speak, but any time it spends at room temperature, its lifespan is ticking away, so keep it out of the freezer for the shortest possible time. Acidic-based mixes (fruit juice, amla, or citric acid mixes) have a longer lifespan in general, so they will also survive repeated thawing better than more pH neutral (water, weak tea) mixes.
Cassia paste can be frozen and will lose only a portion of its dye strength, but it isn't very strong to start with. In a henna/cassia mix that is frozen, you probably will not notice a difference in your color outcome using frozen paste versus fresh because cassia wasn't playing a huge role in the color in the first place. If you are using cassia by itself to dye white hair, however, you may notice your color comes out a bit lighter blonde in comparison to fresh cassia paste. It is not known by this writer how long cassia dye will survive in the fridge.
Buxus and indigo paste do not survive freezing well. Plan to replace that portion of your mix with freshly mixed dye after you thaw out your henna/buxus or henna/indigo paste. It is not known by this writer how long buxus or indigo paste will survive in the fridge. They don't last as long at room temperature as henna paste does, and are strongest in the first 3 hours or so.