Posted by: Footler PJ20 MAR 2013
A blacksmith is a regular customer at a local pharmacy that still stocks a range of chemicals for sale. While working there one day I asked him what he intended to do with the items he had just requested. Some 20 minutes later I had gained a crowd of restless customers and had learnt that the concept of secundum artem, virtually gone from pharmacy, lives on in his workshop.
The blacksmith explained that alongside his usual wrought iron and steel work he also made decorative artifacts and small sculptures in copper, bronze or brass. These were usually finished with patinas of various types, which he produced using simple chemicals such as copper sulphate, ferric nitrate, ferric chloride, ammonium chloride and potassium permanganate.
He listed some examples of his work. Florentine brown is a traditional Italian finish applied using a solution of ferric chloride and ferric nitrate in distilled water. The article is rinsed with cool water after the colour appears and then dried with newspaper. (He was most insistent that newspaper be used for drying.) The surface is burnished with steel wool then left overnight. The process can be repeated for a deeper tone and then waxed to darken and set the colour.
By using similar methods a mixture of sodium chloride and ammonium hydroxide gives a purple patina initially but changes to light green after additional applications, and a hot solution of potassium permanganate imparts a variegated golden brown finish to cast bronze and brass.
The blacksmith also mentioned that he sometimes packed small steel items in a box full of fallen leaves and wood shavings, whereupon the tannins present would eventually produce a nicely variegated finish. In all cases the metal must be thoroughly cleaned and degreased before starting the process. I gained the impression that the results of his work owed as much to skill and judgement as to precise formulae and timing.