Fig. 10
The glazing or enamel mills are shown in Fig. 10. These mills consist of a strong iron frame securely bolted to a stone foundation. In the sketch shown the framing carries 2 mills, but 3 or 4 can be arranged for. A common arrangement for small factories consists of 2 large mills, and 1 smaller mill, driven from the same shaft. One of the mills is used for foundation or gray mixings, the second for white, and the smallest one for colored mixings. In these mills it is essential that the construction is such as to prevent any iron fitting coming into contact with the mixing, for, as has already been explained, the iron will cause discoloration. The ground plate is composed of quartz and is immovable. It is surrounded by a wooden casing—as shown at a—and bound together by iron hoops. The millstones are heavy, rectangular blocks of quartz, called “French burr stone,” and into the center the spindle, b, is led. The powdered material mixed with about three times its bulk of water is poured into the vats, a, and the grinding stones are then set in motion. When a condition ready for enameling has been reached the mixture is run off through the valves, c. Each mill can be thrown out of gear when required, by means of a clutch box, without interfering with the working of the others. The grinding stones wear rapidly and require to be refaced from time to time. To avoid stoppage of the work, therefore, it is advisable to always have a spare set in readiness to replace those removed for refacing. The composition of the stones should not be neglected, for, in many cases, faults in the enamel have been traced to the wearing away of stones containing earthy or metallic matter. {302}
Enamel Mixing.
The mixing can be done by thoroughly stirring the various ingredients together, and a much better and cheaper system is mixing in rotating barrels or churns. These are mounted on axles which rest in bearings, one axle being long enough to carry a pulley. From the driving shaft a belt is led to the cask, which then rotates at a speed of from 40 to 60 revolutions per minute, and in about a quarter of an hour the operation is complete. The cask should not exceed the 5-gallon size, and should at no time be more than two-thirds full. Two casks of this kind give better results than one twice the size. The materials are shot into the cask in their correct proportions through a large bung hole, which is then closed over by a close-fitting lid.
Mixings.
| I.— | Almost any kind of glass | 49 per cent |
|---|---|---|
| Oxide of lead | 47 per cent | |
| Fused borax | 4 per cent | |
| II.— | Glass (any kind) | 61 per cent |
| Red lead | 22 per cent | |
| Borax | 16 per cent | |
| Niter | 1 per cent | |
| III.— | Quartz | 67.5 per cent |
| Borax | 29.5 per cent | |
| Soda (enameling) | 3 per cent |
The above is specially adapted for iron pipes.
| IV.— | Frit of silica powder | 60 per cent |
|---|---|---|
| Borax | 33 per cent | |
| White lead | 7 per cent |
Fused and then ground with—
- Three-tenths weight of silica frit.
- Clay, three-tenths weight of silica frit.
- Magnesia, one-sixth weight of white lead.