Uses

Zinc is much used for roofs, for light gutters and pipes, for cisterns, chimney pots, ornaments, ventilators, etc.; for slating nails, for tubing, and for covering iron to protect it from oxidation. It also forms a component part of several useful alloys, and the oxide of the metal is used as a basis for zinc paint.

Ores

The metal is produced from the ores known as "calamine" (the carbonate), "blende" or "blackjack" (the sulphide), and red zinc ore (the oxide).

The ore is roasted, mixed with charcoal, and heated in peculiar retorts. The zinc is converted into vapour, condensed, and then fused. Most of the zinc used in this country comes from Belgium.

Properties

Zinc is easily fusible. Cast zinc is brittle when cold. If pure it becomes malleable at about 220° F., and can be rolled into sheets, which retain their malleability. At very high temperatures, such as 400° F., it becomes very brittle again.

The presence of lead makes zinc too brittle to roll at any temperature. Zinc should be cast at a low temperature, or the metal will become very hard, and some of it will pass off in vapour.

Zinc is easily acted upon by moist air; a film of oxide is soon formed, which, however, protects the metal from further action.

If, however, the air contains acid, as it does near the sea and in large towns, the zinc is destroyed.

Soot is very destructive to zinc, forming with it a galvanic couple, which is brought into action by the moisture and acid in the air.1

Good sheet zinc is of an uniform colour, tough, and easily bent backwards and forwards without cracking. ' Inferior zinc is of a darker colour than the pure metal, and of a blotchy appearance, caused by the presence of other metals, which set up a galvanic action and soon destroy the zinc.

1 Proceedings Inst. Civ. Eng. vol. xxvii.

There is no practical engineer's test for the quality of zinc. Good zinc should, however, be as free from iron as possible. The following is an analysis of Vielle Montagne zinc, which shows that it is practically pure : -

Zinc

0-995

Iron

0.004

Lead, etc.

0.001

1.000

Zinc containing more than about 1 per cent of lead should be rejected.

Market Forms.1 - Zinc is sold in sheets 7 feet by 2 feet 8 inches, 7 feet by 3 feet, or 8 feet by 3 feet, described by their thickness and weight in ounces per foot superficial (according to a special gauge which varies with different manufacturers).

Zinc Gauge

The following Table shows the weight of zinc per square foot for the various numbers of the Zinc Gauge, properly so called. This gauge originated in Belgium, and is sometimes called the Belgian Zinc Gauge, but it is known in the trade as the Zinc Gauge, and is used by Messrs. F. Braby and Company, the English agents of the Vieille Montagne Zinc Company, whose zinc, obtained from mines in Belgium, Sweden, and Spain, is of excellent quality and extensively used in this country.

The thickness of the sheets is also given in the Table; those from Nos. 10 to 21 (except 18) have been accurately measured and kindly furnished by Messrs. Braby; the others are calculated.

Gauge.

Approximate Weight per square foot.

Approximate Thickness.

Lbs.

Oz.

Dr.

Inch.

1

0

1

2

.0013

2

0

2

4

.0036

3

0

3

7

•0055

4

0

4

9

•0073

5

0

5

11

•0091

6

0

6

14

•0110

7

0

8

0

•0128

8

0

9

2

•0146

9

0

10

5

•0165

10

0

11

7

•0180

11

0

13

5

•0217

12

0

15

2

•0254

13

1

0

15

•0290

Gauge.

Approximate Weight per square foot.

Approximate Thickness.

Lbs.

Oz.

Dr.

Inch.

14

1

2

12

•0326

15

1

5

12

•0364

16

1

8

12

•0400

17

1

11

11

•0437

18

1

14

11

•0478

19

2

1

11

•0509

20

2

4

10

•0581

21

2

8

2

•0728

22

2

12

14

.0764

23

3

1

1

•0800

24

3

5

3

•0896

25

3

9

5

•0992

26

3

13

7

•1088

Of the above sheets, Nos. 1 to 5 are rolled only to order and of special dimensions. The remaining gauges are made in sheets of all the three sizes mentioned above.

There are several other zinc gauges given in various Price Books, etc., but they are generally based upon the above, the range of numbers being smaller and the weight not so accurately given.

1 The commercial name for zinc before it is converted into sheet and other useful forms is Spelter.

The thicknesses of zinc recommended for roofing purposes are given at p. 273, Part II.

The expansion and contraction of this metal with changes of temperature are greater than of any other, and should be carefully guarded against by laying sheets on roofs without rigid fastenings as described in Part II.

Zinc should not be allowed to be in contact with iron, copper, or lead. In either case voltaic action is set up, which destroys the zinc. This occurs especially, and more rapidly, when moisture is present.

Zinc should also be kept clear of lime or calcareous water, and of any wood, such as oak, which contains acid.

Zinc laid on fiats or roofs where cats can gain access is also soon corroded.

An objection to zinc for roofs is that it catches fire at a red heat and blazes furiously.1