]o April, \gi2.] Farm Blacksjiiithiiig. 219 



-colour of its surface will be noticed. These changes are due to the oxida- 

 tion of the surface of the metal exposed to the atmosphere and heat. As 

 the temperature rises the colours darken. The first indication of change 

 takes place when the temperature rises to 250 deg. Fahr. It will then be 

 noticed to be of a dirty white colour. As the heat increases it assumes a 

 yellow colour, then dark straw, brown, purple, and varying .shades of blue. 

 'J'he changes, which are called the tempering colours, are followed as 

 guides in determining the hardness of the steel. 



The first essential to the production oi good tools is the selection of 

 suitable steel. Manufacturers of steel make it in many grades, and as 

 each grade is intended for a specific purpose if steel be u.sed for a purpose 

 that it is not intended for then good results cannot be obtained. Notwith- 

 standing this there are many who have an idea that the high priced qualities 

 are the best for all purposes. That is a mistake; there are times when the 

 cheapest is best. In purchasing steel it is always well to state the purpose 

 for which it is to be used and the merchant will know what grade to supply. 



The tools about a farm do not as a rule require to be made of high- 

 grade steel, e.g., steel containing a large percentage of carbon. The quality 

 used for miners' drills would suit most purposes and give very good results 

 if properly treated. It costs about 4d. per lb. 



In working steel, greater care must be exercised than in forging iron or 

 mild steel It will not permit of beiiig heated to a temperature above bright 

 red without injury, because when so heated the carbon becomes burnt out 

 and so it is destroyed. Neither must it be hammered after the redness 

 has. left it. for such treatment would cause it to crack. Low grade steel 

 may be welded, but the operation requires considerably more skill than the 

 welding of iron. Welding steel should therefore be avoided, excepting 

 in the case of pointing picks, crow-bars, or plough-shares. What is known 

 as blister steel is most suitable for welding purposes. 



Forging a Cold Chisel. 



To make a cold-chisel (Fig. 54) for cutting metal, take a piece of f-in. 

 octagonal steel, and cut off a piece 6 J in. long. Heat one end to redness 

 and form the head, which is done simply with the hand hammer. In draw- 

 ing out the chisel end, do so with as few heats as possible, for the rea.son 

 that each heating reduces the carbon value. For ordinary purposes the 

 shape of the chisel should be as shown in the sketch. The width of blade 

 does not require to be more than \ in., and the lengih of tapered part 

 alx>ut 3 in. It is now ready for tempering and grinding. 



To temper, heat the tool slowly and evenly to a low red for a distance 

 of about 2 in. from the point, place about i in. of the heated portion in 

 water which has previously been heated to about 70 deg. Fahr., and after 

 a period of about five seconds, slowly raise the tool about \ in. ; let it 

 remain there for several seconds longer. Remove from the water, and with a 

 piece of sandstone, brick, or emery cloth rub vigorously to remove the scale 

 from the surface, when the part which was cooled will be white, and as the 

 heat is conducted to the point from the back so the colours will appear on 

 the surface. When the point assumes a purple hue, quickly plunge again 

 into the water; cool right out, and grind. In grinding a chisel to cut 

 iron the angle formed at the point should not be le.ss than 70 degrees. If 

 ground too thin it will not stand up to the work, no matter how well it may 

 be tempered. 



The punch (Fig. 55) needs no further comment than that it requires the 

 same treatment as the chisel. 



