This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The appearance of stone is often a matter of importance, especially in the face work of conspicuous buildings. In order that the appearance may be preserved, a good weathering stone should of course be selected, free from flaws, clayholes, etc. All varieties containing much iron should be rejected, or they will be liable to disfigurement from unsightly rust stains caused by the oxidation of the iron under the influence of the atmosphere. Stones of blotched or mottled colour should be regarded with suspicion. There is probably a want of uniformity in their chemical composition, which may lead to unequal weathering.
In order to obtain the best stone that a quarry can furnish, it is often important that it should be taken from a particular stratum. It frequently occurs that in the same quarry some beds are good, some inferior, and others almost utterly worthless for building purposes, though they may all be very similar in appearance. To take Portland stone as an example. In the Portland quarries are 4 distinct layers of building stone. Working downwards, the first bed of useful stone that is reached is the True or Whitbed Roach - a conglomerate of fossils which withstands the weather capitally. Attached to the Roach, and immediately below it, is a thick layer of Whitbed - a fine even-grained stone, one of the best and most durable building stones in the country; then, passing a layer of rubbish, the Bastard-Roach, Kerf, or Curf is reached, and attached to it is a substantial layer of Basebed. The Bastard-Roach or Basebed-Roach and the Basebed are stones very similar in appearance to the True Roach and Whitbed; but they do not weather well, and are therefore not fitted for outdoor work. Though these strata are so different in characteristics, the good stone can hardly be distinguished from the other even by the most practised eye. Similar peculiarities exist in other quarries.
It is therefore most important to specify that stone from any particular quarry should be from the best beds, and then to have it selected for the work in the quarry by some experienced and trustworthy man.
Nearly all stone is the better for being seasoned by exposure to the air before use. This seasoning gets rid of the moisture, sometimes called "quarry sap," which is to be found in all stone when freshly quarried. But in hot climates it is sometimes an advantage to retain the quarry sap, fo.
(1) it makes the stone easier to cut, an.
(2) it prevents the moisture from being sucked out of the mortar. Unless this moisture (in cold climates) is allowed to dry out before the stone is used, it is acted upon by frost, and thus the stone, especially if it bo one of the softer varieties, is cracked, or, sometimes, disintegrated. The drying process should take place gradually. If heat is applied too quickly, a crust is formed on the surface, while the interior remains damp, and subject to the attacks of frost. Some stones, which are comparatively soft when quarried, acquire a hard surface upon exposure to the air.