Note 152 — Appendix. Cement for Earthenware. Restorations.

The zinc, which in the course of the reduction process may become coated with a thin layer of metallic copper, may be used again. It should be first put through dilute sulphuric acid (in the proportion of 1:2), then washed, rubbed with a steel wire brush and again washed. But it must in such cases be used again while still wet, for if allowed to dry it becomes coated with a layer of oxide and requires to be re-polished.