How to dry hash: the 65% threshold and the jar test

"Drying" means different things for different extracts, and the deciding factor is how the resin was separated from the plant. Ice water methods leave a waterlogged paste. Dry methods start from flower that has already been dried and add no water.

Rosin causes the most confusion. Nobody leaves it to be "dried" for weeks: it is pressed from hash or flower that is already dry, and what some call its curing takes place cold, in a closed container. Our article on storing rosin, wax and crumble goes into this. From here on, the article deals with ice water hash, the only extract with a real water problem.

Touch is a poor guide to moisture. Laboratories use water activity, written aw, which is the share of "free" water available to microorganisms. The US Food and Drug Administration defines it as a ratio of vapour pressures: the vapour pressure of the product in balance with the surrounding air, divided by that of pure water. The scale goes from 0 (bone dry) to 1 (pure water).

Hemp's reference is the ASTM D8197 standard, written for dried flower, which sets a water activity of 0.55 to 0.65. Mould has trouble growing below 0.65, and above 0.55 the flower holds together when handled. Past 0.68, the standard calls for more drying. We found no equivalent standard for hash, so we use these benchmarks by analogy: resin is also plant material, and rich in trichomes.

1. Squeeze out the free water: press the resin gently in the filter bag, then on paper towel, until nothing more runs off.
2. Crumble it. Fine grains lose their water faster and more evenly, while a compact ball dries on the outside and stays wet in the middle.
3. Spread the crumbs thinly on baking paper or a glass plate, with no heaps.
4. Leave it all in a cool, dark room where the air circulates without blowing directly on the resin. A fan pointed at it blows the powder away and dries the surface too quickly.
5. Once or twice a day, stir and crumble again, until the grains no longer stick together.
6. Before storing anything, run the jar test described below.

How long it takes depends on layer thickness, grain size and room humidity, and it is a matter of days. Trust the jar test over any calendar. Professionals often use a freeze dryer, which removes water under vacuum at low temperature. A 2024 study on flowers found that freeze drying preserves acidic cannabinoids better than tray drying, while losing more of the volatile compounds behind the aroma.

If you press your own resin, our guide to homemade hash covers both extraction methods, the dry one and the ice water one.

Heat is the most frequent mistake, since it looks like a shortcut. It drives off terpenes, which are volatile molecules, and it speeds up the transformation of cannabinoids. In 2026, the University of Mississippi published a stability study of flowers, extracts and isolates kept at room temperature, in the fridge and in the freezer. It found that at room temperature, THC broke down into CBN, while CBD-only products stayed stable. Everything kept best in the freezer at -20 °C.

Time has the same effect. Researchers followed 150 resins seized in Morocco and stored for up to eight years, and measured THC falling from 35% to under 3% after two years as CBN rose. Those resins were rich in THC; the residual THC in a hemp resin oxidises by the same process.

Moisture, last, feeds mould. An Israeli team compared controlled-atmosphere drying chambers with traditional flower drying. In their results, traditionally dried flowers were heavily infested with Alternaria alternata, and Botrytis cinerea was present at lower levels. Resin that is wet, compact and poorly ventilated gives these fungi the same conditions.

The test follows directly from the definition of water activity. Inside a closed container, product and air reach a balance, and the air's relative humidity divided by 100 then equals the product's water activity. The ASTM standard also works from the air inside the container.

1. Fill a small glass jar about two thirds full with a sample of resin.
2. Slip in a small digital hygrometer and close the lid.
3. Leave it for 24 hours or more, in the dark and at a steady temperature.
4. Read the hygrometer. Above 65%, the resin goes back to dry; between 55 and 65%, it is ready to store. Under 55% it is very dry, which is fine for keeping, though it crumbles more.

Cheap hygrometers can drift by several points, so check yours first. The salt test does the job: seal the device for 24 hours with a spoonful of moistened table salt, which holds the air at about 75% relative humidity. Run the jar test at room temperature too, since water activity changes with temperature. Resin that smells musty or shows white or greyish fuzz should not be used; our article on mould shows how to recognise it.

When the reading is under 65%, move the resin to a closed glass jar kept away from light. In the 2024 flower study cited above, glass jars kept aromas better than a plastic box or an open tray. Cold slows breakdown over long periods: use the fridge, or the freezer for the most fragile ice water hashes. Let the jar return to room temperature before you open it, so that no condensation forms.

If the resin turns out slightly too hard or too soft after drying, it can still be saved. Our article on tempering hash explains how to soften it without heat. The resins in our CBD hash range are sold already dry.

Under French law, a hemp resin may only be sold if its Δ⁹-THC content does not exceed 0.30%, the threshold set by the order of 30 December 2021. The Conseil d'État annulled on 29 December 2022 the ban on raw flowers and leaves contained in that order. Our article on whether CBD hash is legal goes through the three texts.