Metalsmithing techniques have changed little since man discovered fire. To make a copper bowl, metalsmiths have always begun by using a sinking hammer to form the copper and then used a planishing hammer to pound out tool marks. Torches now offer more heat control than did early torches. Hammers are more specialized. However, when you make a copper bowl, you still follow a tradition that dates back to early man.
Prepare the copper disk for sinking. Anneal (heat) the copper disk with the torch. Place in pickle (a solution that eats away at the oxidation caused by the torch), rinse with water and dry. Use the compass to mark five concentric circles evenly spaced.
Begin sinking the bowl. Place the marked disk on the sandbag, dapping block or carved wood block so that it sits on an angle. Hit the outermost marked line with your sinking hammer all the way around the disk. Move to the next line and repeat until you have formed a 1 inch lip all the way around the outer edge of the copper disk. Stop from time to time and anneal the metal as you work.
Prepare to planish the bowl. Turn the bowl over and draw a second set of evenly spaced concentric circles on the bottom of the bowl.
Place the copper bowl on the mushroom stake. Locate the circle that is about midway on the bottom of the bowl. The midway circle will mark the spot where the copper disk begins to curve to create the lip of the copper bowl. Hit the copper disk on the line with your planishing hammer all the way around the disk. The planishing hammer will create a smoother, more pleasing curve for your bowl. Continue to work until you planished away all unsightly tools marks caused during sinking. Be sure to continue moving the planishing hammer around the metal; if you hammer more on one side, the bowl will be uneven. Adjust the contour of the bowl as needed while you work. Stop to anneal and pickle the metal from time to time.
File the edges of the copper bowl until they are smooth. Take care that the bowl is still round after filing.
Check that the bowl sits flat. If the copper bowl rocks, correct it with your planishing hammer and files.
Sand using sequentially higher grits of wet/dry sand paper until there are no tool marks; you have removed all firescale; and the copper bowl is highly polished. You will need 220, 320, 400, 600 and 800 grits of wet/dry sand paper.
Buff and polish the copper bowl using a three-step process. Begin by placing a stitched muslin wheel on your polishing machine. Add tripoli polishing compound to the wheel and polish the inside and outside of the bowl. Wash the bowl with water and detergent to remove all traces of tripoli. Repeat using white diamond and a fresh muslin wheel. Clean and repeat again using red rouge. Take care that there is no cross contamination of the three different polishing compounds and the three muslin wheels used.
Metalsmithing involves use of a torch and numerous chemical compounds. Be sure to work in a well-ventilated space. Always take fire safety precautions when using a torch. Wear eye protection when working with chemicals. Wear a full face shield when working with a polishing machine.