20. PREPARATION OF IRON FOR COATING WITH COPPER When it is required to cover an iron article with copper, it is first steeped in hot caustic potash or soda to remove any grease or oil. Being washed from that it is placed for a short time in diluted sulphuric acid, consisting of about one part acid to 16 parts of water, which removes any oxide that may exist. It is then washed in water and scoured with sand till the surface is perfectly clean, and finally attached to the battery and immersed in the cyanide solution. All this must be done with despatch so as to prevent the iron combining with oxygen. An immersion of five minutes duration in the cyanide solution is sufficient to deposit upon the iron a film of copper, but it is necessary to the complete protection of the iron that it should have a considerably thick coating, and as the cyanide process is expensive, it is preferable when the iron has received a film of copper by the cyanide solution, to take it out, wash it in water, and attach to it a simple cell or weak battery, and put it into a solution of sulphate of copper. If there is any part not sufficiently covered with copper by the cyanide solution, the sulphate will make these parts of a dark colour, which a touch of the finger will remove. When such is the case, the article must be taken out, scoured, and put again into the cyanide solution till perfectly covered. A little practice will render this very easy. The sulphate solution for covering iron should be prepared by adding it by degrees a little caustic potash, so long as the precipitate formed is re-dissolved. This neutralizes a great portion of the sulphuric acid, and thus the iron is not so readily acted upon. When the iron is thus coppered, proceed to silver it in the manner recommended for silvering according to receipt No. 9; or if you want to put a very heavy coating of silver on it, make use of a strong battery. 21. SOLDERING FLUID For mending articles of tin, iron, zinc, copper, and almost all other metals. Take 2 fl. ounces of muriatic acid, add zinc till bubbles cease to rise, add 1/2 a teaspoonful of sal-ammoniac and 2 ounces of water. Damp the part you wish to solder with this fluid, lay on a small piece of lead, and with a piece of hot iron or soldering iron solder the part. 22. SOLDER FOR TIN Take of pewter 4 parts, tin 1 part, bismuth 1 part; melt them together. Resin is used with this solder. 23. COLD METHOD OF SILVERING IRON WITH SILVER-PLATE