A slight addition of a weak solution of pearl ash, 4°-8° Tw.
(20° B.), when the calcium sulphate is allowed to deposit, and the separated liquor concentrated to 48° Tw.
The chemicing is best done cold and with weak solution, at ½° Tw.
Caustic soda (sodium hydrate) comes into commerce in a liquid form as 90° Tw.
The souring is done by passing the goods through an acid liquor at 2° Tw.
