THE SCAIFE

In early times the scaife, as the polisher's rotating wheel is called, was a horizontal disc of cast iron, carefully balanced and supported horizontally on a wooden axle with pointe' ends going into conical bearings drilled into hard wood. (This arrangement is still favoured by many of the fines craftsmen.) This scaife was driven round at the respectabl speed of some 2000 revolutions a minute by means of a rope belt pulley arrangement, rotated from quite a large hand-driven horizontal wheel, maybe of some 5 feet in diameter. A big ‘gear ratio’ was used to attain the high speed of rotation. This was not really difficult, for a 30:1 ratio requires only about one revolution per second of the big wheel. It was the prolonged action not the speed which was so very strenuous, and old prints show that the unfortunate wife or daughter o the craftsman was employed on the deadly arduous task of keeping the scaife on the turn, probably for hour after hour.

The scaife was first scored with a file and on it was rubbed a mulch of graded Diamond powder mixed with olive oil. There existed certain guild secrets about oil preparation, and it was maintained that a red-hot brick dropped in the oil (oil of brick!) improved performance. In this connection it is of interest to note that modern operatives often use a past made of Diamond grit mixed with grease, but improved performance is obtained with these pastes, when the manufac­turer incorporates some organic detergent. There is a likelihood that the mysterious oil of brick might contain, from the heat treatment, organic compounds with a detergent action, so that the old guild secret trick might well have really forestalled modern technique. The craftsman made his Diamond powder by crushing off-cuts and waste with a pestle and mortar, using the gritty feel as a test of the size of the particle he required.

A secret of the polishers’ guild, which was passed on from father to son, was which were the 'soft' and which were the 'hard' directions on the Diamond being worked. Even today many polishers are darkly mysterious about this, although the facts are now widely known. The Diamond was mounted into a curious device, called a dop and tang, shown in Fig. 9. This is still in use today, unchanged. The dop in Fig. 9 is the acorn-like holder into which the Diamond was fixed and held with a blob of solder or lead. The Diamond remains quite unharmed by being fixed with molten solder. Some modern dops use mechanical clamp devices, but the best craftsmen still use a solder dop. The tang is made of wood, a tripod, with two wooden feet and a somewhat flexible stalk of thick copper wire, which holds the dop, the third foot. The flexible wire enables the operative to tilt the dop (and the Diamond) to a required angle for polishing a selected plane direction. On the tang a lead weight (a pound or two) is placed. The scaife is rotated and the Diamond gently swept over it, steadily, and ever so slowly it is ground away. The angles are set by eye, with astonishing accuracy by the good operative.

Increasing speed or pressure is of no avail. On the contrary, 'burns' and score marks can than appear. This is because a local rise in temperature can take place if pressure is excessive and the Diamond actually begins to graphitise on the surface, leading to black streaks. Time taken for polishing a small face may be hours, a large face can occupy days. Really formidably large Diamonds (those in the class of 100 carats) take many months of continuous polishing to reduce them down to required shape. It is for this reason that shape affects the value of rough Diamonds prior to polishing.

These old techniques are still employed today, but, of course, scaifes are driven by fast electric motors, at speeds of about 4000 to 5000 rev/min. For the manufacture of high-grade GEMS the old classical medieval scaife is still used, and nothing else; but for mass production of polished Diamond tools a considerable increase in speed of operation has resulted from the introduction of the Diamond-bonded wheel, of which more anon.

wire saw was rubbed back and forth over the Diamond. The J method was exceedingly slow and it is on record that the J sawing in half of the celebrated Regent Diamond (410 carats) took a whole year of continuous work, and incidentally used up a good deal of grit.

The Diamond slitting saw is a thin disc of bronze, of thick- * ness perhaps 1/200 inch, of diameter some 3 inches. It is held between thick strong disc flanges to stiffen the bronze disc, such that only perhaps half an inch of the thin bronze disc protrudes. This bronze disc is driven by a motor at about 4000 rev/min or a little faster.

The Diamond to be cut is held on an arm above the wheel and rests on the wheel, i.e. a ‘gravity feed’ is used. Sawing must be carried out on soft planes and in specified directions on such planes, for in both polishing and sawing a selected soft plane has also favoured directions which operatives call the ‘grain’. Sawing or polishing times are reduced if correct the grain directions are exploited and better ultimate finishes aril also secured.

Demands on the bearings and smoothness and trueness of rotation are high, for if the slit cut widens, there is excess loss of weight in the Diamond being cut and more and more time is needed for the work. The sawing is activated by charging the edge of the bronze disc with a roller coated with a mulch of fine Diamond grit and olive oil. Frequent recharging is necessary. Special precautions must be taken to avoid vibrations, and saws are usually bedded firmly on concrete. Of course, the whole outfit is quite small.

With modern machines such a saw takes two hours to cut| across a 1-Carat crystal. Large Diamonds may take several days of continuous operation. Great difficulties are experienced where the Diamond is twinned. When this happens, the saw, whilst traversing the soft plane, suddenly meets a fiercely hard twin edge and this is so very hard that sawing is brought to a standstill. These twinning conditions are dreaded in the industry, such a twin plane being called a ‘naat’. There is al tradition that Diamonds from Australia cannot be sawn. This is really because they have regions of repeated twinnings