The container has one job
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Why a bath bomb fizzes
Citric acid and sodium bicarbonate react in the presence of water. That is the product working. The same reaction proceeding slowly in a jar is the product spending itself before it is used.
Which makes the liner the component to specify
Of the 257 closures on this shelf, 61 carry an HIS induction-seal liner, 38 F217, 30 pressure-sensitive and 25 none at all. A closure with no liner on a hygroscopic reactive product is the wrong starting point.
Gasket or membrane, again
A foam or F217 liner seals while the cap is on and tight. An induction liner bonds to the land and stays sealed with the cap off, which is a materially better moisture barrier for a product held over months.
The desiccant is a consumable
With a capacity, sized to the headspace, absorbing from the moment it is exposed. One left open on a bench before packing can be saturated before the jar is closed, which is a handling failure rather than a specification one.
Headspace is the variable you control
A part-filled jar holds air and air holds water. Sizing the container to the fill rather than part-filling a larger one makes the liner and the desiccant both work less hard, and it looks better.
Humidity in the workshop counts too
A bath bomb pressed and jarred on a humid day arrives with moisture already inside. Curing properly before packing, and packing in the driest conditions available, is free and effective.
Salts cake rather than react
Bath salts, soaks and milk baths take up water and bridge into a solid mass. Same cause, same controls, different symptom, and a clear jar shows it immediately.