Crystals of iron pyrites are often found in slates, especially in those from Scotland, etc.

They are often considered objectionable. It should, however, be borne in mind that there are two varieties of pyrites, of the same chemical composition but of different crystalline form, and very different in their resistance to atmospheric influence.

Ordinary Iron Pyrites, consisting of yellow brassy crystals, generally cubical, weathers well. The crystals have been found perfectly bright and firm in their places in roofs 100 years old, even in the atmosphere of Glasgow.

White Iron Pyrites (or marcasite), on the other hand, is easily decomposed, and slates containing it ought to be rejected. This form of pyrites is generally dull and wanting in lustre, and is therefore not easily seen.

Sizes

The slates sent to the market are squared in the quarry - sometimes roughly by hand, sometimes by machinery- to certain sizes, which are distinguished by different names,1 as shown in the following Table.

In buying and selling slates in this country, a "thousand" is generally understood to mean 1200 or 1260.

The Table shows the weight and cost of 1200 slates of each description; it also shows the number of slates required per square (100 superficial feet) of roofing, and the weight per square.

1 These names are used in the building trade, but not much in the quarries.

The following Table of the Different Slates in use has been taken from the list of the Oakeley Slate Quarries Company (Jan. 1887) for slates supplied at Portmadoc. The rates vary of course according to the state of the market, though the prices show the relative value of the different kinds.

NAMES.

Sizes.

First Quality.

Medium Quality.

Sizes.

Second Quality.

Sizes.

Third Quality.

Allowing 8 inch lap will cover, about

Computed Weight.

Price per M of 1200.

Computed Weight.

Price per M of 1200.

Computed Weight.

Price per M of 1200.

Computed Weight.

Price per M of 1200.

T.

c

Q.

s.

d.

T.

c.

Q

s

d.

T.

c.

Q

s.

d.

T.

c.

Q.

s.

d.

Empresses. . . .

26x16

4

0

0

15

15

0

4

0

0

14

0

0

..

..

..

..

...

..

..

..

...

..

..

..

..

170 Sq. Yds.

Small Empresses .

26x15

3

15

0

15

0

0

3

15

0

13

5

0

..

..

..

..

..

..

..

..

..

..

...

..

..

160 do.

Princesses . . .

24x14

3

5

0

13

10

0

3

5

0

11

0

0

24 xl4

4

0

0

9

0

0

..

..

...

..

..

..

..

136 do.

Duchesses . . .

24x12

2

15

0

11

0

0

2

15

0

10

0

0

24 xl2

3

10

0

7

10

0

..

..

...

..

..

..

..

116 do.

Small Duchesses .

22 x 12

2

10

0

9

0

0

2

10

0

7

15

0

22 xl2

3

5

0

6

7

6

..

..

..

..

....

..

105 do.

Marchionesses . .

22x11

2

5

0

8

2

6

2

5

0

7

0

0

22 x l1

3

0

0

6

0

0

..

..

...

..

..

..

..

97 do.

Countesses . . .

20x10

1

15

0

6

15

0

1

15

0

6

5

0

20 x l0

2

7

2

5

0

0

20

2

10

0

4

0

0

78 do.

Wide Viscountesses

18x10

1

12

2

5

7

6

1

12

2

4

17

6

18 xl0

2

2

2

4

0

0

. . .

..

..

..

..

..

..

69 do.

Viscountesses . .

18 x 9

1

7

2

4

15

0

1

7

2

4

5

0

18 x 9

1

17

2

3

5

0

18

2

0

0

3

0

0

62 dc.

Wide Ladies . . .

16x10

1

7

2

4

10

0

1

7

2

4

2

6

16 xl0

1

17

2

3

5

0

..

..

. •.

..

..

...

...

60 do.

Broad Ladies . .

16 x 9

1

5

0

4

0

0

1

5

0

3

12

6

16 x 9

1

12

2

2

17

6

• • •

..

..

..

..

..

..

54 do.

Long Ladies . . .

16 1/2x8 1/2

1

5

0

4

0

0

1

5

0

3

12

6

161/2 x 81/2

1

12

2

2

17

6

16

1

15

0

1

17

6

52 do.

16 x 8

1

2

2

3

12

6

1

2

2

3

5

0

16x 8

1

10

0

2

7

6

• • •

...

..

..

.

....

..

48 do.

Wide Headers . .

14x12

1

10

0

4

7

6

1

10

0

3

17

6

14 xl2

1

17

2

3

5

0

..

..

..

..

....

...

..

60 do.

14x10

1

5

0

3

12

6

1

5

0

3

5

0

14 xl0

1

15

0

2

12

6

14

1

5

0

1

2

6

50 do.

Small Headers . .

13x10

1

2

2

3

5

0

1

2

2

2

15

0

13 xl0

1

7

2

2

5

0

. • •

..

...

..

..

...

..

46 do.

Small Ladies . .

14x 8

1

0

0

2

15

0

1

0

0

2

10

0

14 x 8

1

5

0

1

15

0

13

1

0

0

1

0

0

40 do.

Narrow Ladies .

14x 7

0

17

2

2

5

0

0

17

2

2

0

0

14 x 7

1

2

2

1

10

0

..

..

..

..

..

..

..

36 do.

Doubles ....

13 x 7

0

15

0

2

0

0

0

15

0

1

15

0

13 x 7

1

0

0

1

5

0

• • •

..

• • •

..

..

. .

..

32 do.

Wide Doubles . .

12 x 8

0

17

2

2

0

0

0

17

2

1

15

0

12 x 8

1

0

0

1

5

0

• •

..

.. .

..

..

• • •

..

32 do.

Small Doubles . .

12 x 6

0

14

0

1

5

0

0

14

0

1

2

6

..

..

..

..

..

..

..

Moss Slates 20/

per ton.

25 do.

Singles ....

10 x 8

0

15

0

1

5

0

0

15

0

1

2

6

..

..

..

..

..

..

..

Rags 30/to 40/

per ton.

26 do.

Hexagon, Gothic, Diamond, Round, or: other Fancy Slates made to order at the above juices in First Quality only. Queens - First Quality. - 24, 26, 28, 30, 32, and 34 inches lung and various breadths, assorted as made, without specifying quantities of each, at per ton of 20 cwt. of 112 lbs., 82/6 and upwards. Medium Quality. - As above, 65/ and upwards. Second Quality. - Do. 50/ do.

The weights given are for Portmadoc slates. Slates from Bangor and Fenrhyn are somewhat heavier, and Westmoreland slates heavier still. Rags vary in size, but average 36 x 24 inches. Imperials 30 x 24 inches. Queens, Rags, and Imperials are sold by weight.

' Quality. - The market qualities of slates are classed in the quarries according to their straightness, smoothness of surface, fair even thickness, and in the Cambrian quarries according to the presence or absence of discoloration.

Slates are generally divided into 1st and 2d qualities; in some cases a "medium quality" is quoted.

All slates for good work should be hard, tough, non-absorbent, of uniform colour, free from patches, from veins, iron, cross-grain, and with smooth and even surfaces.

Thickness

The thickness increases with the area of the slate, and the rule for the proportionate thickness varies in different quarries but for Welsh slates is somewhat as follows: - l

THICKNESS.

1st Quality.

2d Quality.

Duchesses and Marchionesses .

...3/16 inch

3/11 inch.

Countesses and large Ladies

• • 1/6 „

1/4 ,,

Doubles ....

TO >>

3/13

The best qualities of Welsh slates generally split easily into even sheets with smooth surfaces, and holding their thickness close up to their edges, even after being squared.

Irish and Scotch slates are often of very uneven thickness, being thicker in the middle than near the edges, and very much stouter and more substantial than Welsh slates of the same area.

Slates are sometimes split too thin, so that they are not strong enough for roofing purposes. The Ffestiniog quarries have produced (for exhibition as specimens of perfection of cleavage) slates 5 feet to 10 feet long, 6 inches to 12 inches broad, only 1/16 inch thick.2

Tests

The following rough tests are generally recommended, but they are not of a practical character, nor can they be relied upon. Experience is required to judge of a slate by the eye.

1. Weigh the slate carefully when dry, steep it in water for 24 hours, run the water off, and weigh again - any difference of weight will show the amount of absorption.

2. Stand the slate in water up to half its height - if it be of bad quality the water will rise in the upper half, but in a good slate no sign of moisture will be seen above the water-line.3

3. Breathe on the slate. If a clayey odour be strongly emitted it may be inferred that the slate will not "weather." 4

1Wray.

2 Hunt.

3Gwilt.

4 Dempsey.