This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Examination for flaws in the stone is, whether specified or not, essential in all cases, and a considerable amount of judgment has to be exercised in accordance with the particular stone which may happen to be specified, and the position in which it is to be used. If an obviously soft and friable stone be supplied for a position where it is likely to be subjected to wear, the attention of the Architect should be called to the matter, even if it complies with the specification as regards the quarry from which it is obtained, and similarly a stone which is chalky or earthy and soft should be regarded with suspicion for external work. Definite sand-holes can generally be seen by inspection, while vent or cracks are to be discovered by tapping the blocks all over carefully with a pebble. So long as a ringing sound is emitted the stone may be accepted; but directly the "voice" of the stone - if it may be so called - becomes dull, it should be examined for a crack, and the faulty part cut out, or else the block be discarded.
It is not often necessary for the Clerk of Works to determine whether any of the stone contains lime. If it be obtained direct from the quarry there is rarely any doubt as to where it comes from; but sometimes this doubt exists if it be obtained through a merchant. Thus "best York stone," which all know to be a practically pure sandstone, would be rejected at once if, on application of a little acid, it were found to effervesce, showing that lime was present. Such terms as " York stone," for instance, have a very wide signification. Some of the sandstone obtained from Yorkshire is of a coarse grit and deep in the bed, while other is of fine grain and comes out in thin slabs; while all colours, from white through cream to brown, and also blue stones, fully answer this description. It even happens occasionally that York stone is supplied which has been quarried in the Forest of Dean; but this is so hard to distinguish from some of the best York stones that it may generally be accepted.
A most difficult thing to detect about some stones is whether they are being laid on their natural bed, as is specified to be done in most cases. The bed is easy enough to discover in most of the laminated stones, such as the sandstone landings, while in other cases it is immaterial, as in the thickly bedded and homogeneous stones. In shelly stones it can be discovered by the direction in which the fossils lie, as naturally they will have been deposited flat on the sea-bed originally. Thus a fossiliferous stone must always have its shells lying down on its side when it is flat bedded. In the oolites the discovery of the bed is difficult, unless there be fossils present, except by a skilled workman, who will know it by the feel of his tool as he works the surface.
A slate which does not ring properly when struck is sure to be faulty, being probably of an earthy character and highly absorbent. Absorption is tested by standing a slate upright, partially submerged in water, when it ought to show no sign of moisture creeping up the edges within half an hour. Some slates will, however, become moist in that time as far as 2 inches above the water-line, and, of course, any such should be rejected, for there should be practically no rise of moisture at all. A bad slate, too, will give out an earthy odour when wetted. Many Architects prefer a slate which splits very thin; but such are by no means the best for roofing purposes, as they are exceedingly liable to crack, and a strong and comparatively thick slate of rough surface is often to be preferred.
Of all the materials supplied upon a building, timber is that upon which the exercise of judgment upon the Clerk of Works' part is most essential. Under the usual specification there is a great deal left to the imagination; or perhaps it would be more proper to say that that which is specified is generally difficult, if not impossible, to obtain. Architects go on specifying Memel, Dantzic, or Riga timber, in spite of political events upon the Baltic, and the facts that the Prussian and Russian forests are nearly worked out, and that a
Duties of Clerks of Works - Testing Materials 135 great deal of so-called Baltic timber is imported from Canada and the United States. This is so nearly allied to that which it was the Architect's intention to demand that it may generally be accepted; while in very many cases it would be impossible to distinguish the one from the other except for the brands. Thus good sound fir may generally be passed for carpenter's work wherever it may happen to come from, provided that it be clean, straight, and not too open in grain, fairly free from knots, and well seasoned - all of which can be seen by inspection. The proportion of sapwood which may be allowed is often a difficult point, upon which it is well to consult the Architect at an early stage. If in the specification it says that the timber shall be absolutely free from sap, there is no question as to its legal meaning; but it may be very difficult to get it, especially in the larger scantlings. Even the smaller scantling stuff, as frequently imported now, is cut from quite young and small trees, and so contains a large proportion of sapwood. Thus if the word "absolutely " be not present in the specification, merely " free from sap " means nothing more than " reasonably free from sap," and the Clerk of Works' position becomes difficult, as the onus of judgment is thrown upon his shoulders.
Any tendency to decay, other than the presence of sapwood or of actual rot, can be detected by tapping the timber and noting the sound given forth, which ought in no case to be dull; while a slight tap with a key at one end of a piece of timber should be distinctly heard if the ear be placed close to the other end. A speckly or dotty appearance is also indicative of incipient decay. Even although the specification may make no mention of it, waney timber, especially if it run out to the bark, may always be rejected, as it necessarily consists almost entirely of sapwood; and so, of course, must warped timber, or that with broken grain, or containing large loose knots.
Joinery work ought always to be inspected in the shops before it has been put together, to make sure that no unseasoned rubbish or sapwood has been used. If it is not seen until it has been brought upon the site there is much greater hardship in rejecting it. Some Builders will do their very best to get the Clerk of Works to accept joinery which is sent on to the work ready primed. The woodwork thus being covered by a thin coat of paint, it cannot be seen whether it contains defects or not, and in no case should this be permitted.
With regard to metalwork, it is a very common practice for Architects to specify tests which they never mean to have applied, such as those for the ultimate breaking weight of the metal. If it be obtained from a good firm, as a general rule the firm must be trusted, as only in large works is it possible to cut test pieces and have them subjected to direct tensile tests. In ordinary building work all that the Clerk of Works has to do with regard to metal is to see that what is supplied is sound. Large castings, for instance, should be carefully tapped all over with a light hammer, in order to detect flaws by means of sound; and it should, of course, be seen that all bolt-holes have been cleanly drilled, and that bearing surfaces are true. Steelwork, in the same way, should be carefully tapped all over, particularly the rivets, in order to discover any that may be loose, which would have to be cut out and replaced by others. The rivet-heads should also be in proper alignment, and whether it be specified that the bearings are to be planed or not, these should all be sufficiently perfect for no wedging to be required. Under a bad specification this may be exceedingly difficult to insist upon; but with the exercise of firmness and tact good work can generally be obtained, even under such conditions. In large works where much steel is used, testing ought to take place with stringency; but it should be done by keeping a representative at the mills, who could have test pieces cut from the various plates and girders before they are built up, and tested at once, rather than on the works.
There are few other materials about which definite directions for testing need be given, as all Clerks of Works of any experience are able, for instance, to discriminate between good sheet glass and that which is wavy, and to detect whether lead of the full weight specified has been supplied. In these days of ready-mixed paints it is not often that adulteration is attempted. It is always best to have paints which have been mixed by proper machinery than those which have been made up by hand; but if the latter be specified or permitted, then it is just as well to test the white-lead by placing a little in a saucer and covering it with nitric acid, which will dissolve the lead and leave any adulterant as a residue.