Calcium sulfate (CAS 7778-18-9): storage and disposal

Plaster arrives in a paper sack or a thin tub, and neither keeps damp out for long. Once a sack has been opened and folded over, the plaster at the top starts taking moisture from the air and stiffens into lumps, and a bag stored near a tap or on a floor that gets mopped can set solid. Move it into a rigid, tight lidded container, separate from the dihydrate, and label both. Keep the pair on a high, dry shelf in a general store, dated on arrival, and apart from food grade supplies. School prep rooms often hold several part-used tubs from different suppliers, so combine nothing and record each by its own label.

Dihydrate looks the same year after year: a fine white powder or small crystals. Plaster is the one that changes. A sack that feels stiff, rattles or has hard nodules inside has taken in water, and powder that no longer runs freely will not behave as a fresh batch. A fully set block in the bottom of a tub is the extreme case. Split paper, a lid stuck down with white crust and a label that has lifted from a damp surface all point to storage that was too humid. Check the date of receipt as well, since undated tubs from earlier years are the usual find.

We arrange this by specialist collection. We do not publish a drain or general waste route for any chemical. It is normally packed in a non-regulated or low hazard laboratory pack with compatible chemicals.

Plaster sets hard when it meets water, which is why this stock must not go down a sink or drain, where it can block pipework. It must not go into general waste or a skip either, and nobody should try to treat it first. Collection is by the specialist route, and waste code 16 05 09 is the usual entry, though the producer classifies the waste. When you describe the stock, say which form each container holds, whether the plaster has already set, and how much is left. A set block in a tub is described as set plaster, not as loose powder. Dihydrate and plaster can travel in the same lab pack if their containers are sound and closed.

Usually held as: dihydrate, plaster.

Tell us the container size and how many you hold. An estimate is enough to start with, and some items cannot be priced until someone has looked at them.