By historic standards, they are still "pretty good", insists Paul Nowak, national organiser of Britain's Trades Union Congress.

The dynamic performance, corrosion rate and coking time of the mortar were tested under normal temperature and after heat treatment and corrosion individually.

The production shows that the best bluing time in normal temperature is from 4 to 8 minutes and bluing film is bright and anti-corrosion capacity is at or higher than the specified state standard.