The reducing agent may include chelates formed of formic acid, citric acid, metasilicic acid, ethylenediaminetetraacetic acid, ethylenedinitrotetraacetic acid or a sodium salt or potassium salt of any of these and a heavy metal such as iron, copper or chromium; and ferrous sulfate or ferrous pyrophosphate. [0127] As the activator, usable are sodium sulfite, sodium hydrogensulfite, formaldehyde sod