AS AFFECTING ARCHITECTURAL AND ENGINEERING WORKS.
The native stones we Liverpool architects have at command are all sandstones belonging to the geological division called the Trias, or, in older phraseology, the “New Red Sandstone,” which lies above the coal-measures. The term “New Red” was given to distinguish these rocks from the “Old Red,” which lies below the Mountain Limestone, the lowest division of the carboniferous rocks. It is, perhaps, needless to remark that the “New Red” is not always red; sometimes it is yellow, at others, like some of the Storeton stone, white. These red rocks occupy a large part of Lancashire and Cheshire, and especially in the latter county give the characteristic scenery which distinguishes it. The escarpment of the Peckforton Hills of which Beeston Castle Hill is an outlier, and that at Malpas, farther south, gives rise to some very beautiful scenery; and again at Grinshill and Hawkstone, in Shropshire, we have a repetition of much the same kind of landscape. It will be necessary for my purpose to say briefly that these red rocks have been divided into the “Bunter” and “Keuper”; the lower division, the Bunter, occupying most of the ground about Liverpool; the upper, the Keuper, being more developed on the Cheshire side. All these sandstones are not fit for building purposes, and those that are so used differ considerably in their durability. It is my object in this short Paper to show upon what the perfection or imperfection of the various stones for building purposes depends—a matter of great moment to an architect or engineer who is desirous that his work should last.
Sandstones, or, in masons’ language, “free-stones,” from the freedom with which most of them are worked when freshly taken from the quarry, are plastic or sedimentary rocks. That is, they are composed of separate particles which have once existed as sand, like that we see on our own shores, or in the sand dunes of Hoylake or Crosby. Sandstones are usually more or less laminated, and are stronger to transverse stress at right angles to their natural bedding than in any other direction, a fact recognized in every architect’s specification, which states “all stones must be laid on their natural bed,” a direction that unfortunately sometimes begins and ends in the specification. The cause of the superior strength is not, however, generally understood.
I have devoted some considerable time to an investigation of the internal structure of sandstones, which I have communicated from time to time to various scientific societies and publications, and will now briefly explain it in a manner I judge will be most likely to interest architects and engineers. The particles or grains of which the rock is built up are of various forms and sizes, from a thoroughly rounded grain, almost like small shot, to a broken and jagged structure, and to others possessing crystalline faces. These grains, most of them possessing a longer axis, have been rolled backwards and forwards by the tides or by river-currents. The larger grains naturally lie on their sides when freshly deposited, with their axes in the plane of bedding; the smaller and more rounded particles naturally tend to occupy the interstices between the others, and in this way rude divisional planes or laminations are formed. Each layer forms a sort of course like coursed-rubble in a wall, and by the necessities of deposition a certain rude geometric arrangement results, by which the particles of the future rock overlap each other, and thereby gain what is known to architects as bond.
But, so far, this is only like “dry walling,” the mass wants cementing together to make it solid. The cementing process happens in this way in our rocks, which are almost purely silicious: Water containing a minute quantity of carbonic acid in solution, which most rain-water does, especially when it comes into contact with decaying vegetation, has the power of dissolving silica to a slight extent. This is proved in various ways, and is shown in the fact that all river water contains more or less silica in solution.
The circulation of water through the sand deposit of which our rocks are made dissolves part of the grains, and the silica taken up is redeposited on others. I cannot explain the chemical reaction that produces this deposition, but that it takes place in the rock during some period of its history is certain. I exhibit a quartzite pebble taken from the Triassic sandstone at Stanlow Point, which, as can be easily seen, was at one time worn perfectly smooth by attrition and long-continued wear, for the quartzite is very hard. Upon this worn surface you will see spangles and facets which reflect the light, and on closer inspection it will be evident that they are crystals of quartz that have been deposited upon the surface of the worn pebble after it became finally enclosed in the rock.
A microscopic examination of the granules of the rock itself will show that many of them have had crystalline quartz deposited upon their surfaces, and in some cases rounded grains have in this way become almost perfect crystals.
An examination of the best sandstones for building purposes shows that they possess more of these crystalline particles than the inferior ones, and a good silicious sandstone shows its good quality by a fresh fracture sparkling in the sun. In addition to these crystalline deposits of silica I believe it exists also as a cement which binds the particles together when in contact.
It certainly is, however, with this secondary silica that the original sand has become a building stone, and the particles have become interlaced and bound together. Thus, in building parlance, the grains are the rubble of the wall, the currents the quarrymen, masons and laborers, and the silicious infiltration the mortar.
And now, when I am on the subject, I may point out that this hard and compact quartzite pebble was also once loose sand. The only difference between the sandstone in which it was imbedded and itself is that in the latter case the process of silicious deposit has gone further, so that all the interstices between the grains have been absolutely filled up with the cement.
It is not possible to see this clearly with the naked eye, but by the aid of a slice of the rock prepared for the microscope the granular structure of the quartzite is made perfectly plain. So much for the mechanical, chemical, and molecular structure of sandstone, all of which affect the strength and quality of the stone; but to architects there is another element of consequence, namely, the color. The rich red of our Triassic sandstones is due to a pellicle of peroxide of iron coating each of the grains. That this is merely surface coloring is shown by the fact that hydro-chloric acid will discharge the color and leave the grains translucent. Unfortunately the most brilliantly colored stone is not the most durable, and it so happens that these brilliant red sandstones are often composed of exceedingly rounded grains. Also some of the very red sandstone has an interfilling of a loose argillaceous irony matter detrimental to the stone as a building stone. The most durable of the red sandstones are those having a paler or grayer hue, like those of Woolton, Everton, and Runcorn. This distinction of color was brought freshly to my mind a short time since in looking at the church of Llandyrnog, in the Vale of Clwyd, a few miles from Ruthin. Some of the dressings, quoins for instance, were of a very brilliant-colored red sandstone, and others of a pale gray or purple red. It struck me that these latter must be of Runcorn stone, which I was afterwards informed was the case. The very red stone was the natural stone of the Vale, originally used for dressings, which were replaced, on the restorations being made, with Runcorn stone. The original stone was æsthetically the best, but the introduced stone the best structurally. The old stone of Chester Cathedral was a very red Bunter sandstone, which decayed badly. It has been replaced in the restorations by Runcorn stone, which belongs to the Keuper division, which has caused the Geological Surveyors to say that the Keuper is a better building stone than the Bunter. In this case it is; but, on the other hand, the Bunter sandstones, or Pebble-beds, as they are called, near Liverpool, are often better than the Runcorn Keuper. The Runcorn building stone lies between two beds of very red loose rock, showing that it is not its geological position, but its structure, that makes it a good durable stone.
It is a remarkable fact that most of the pebbles included in the red rocks are quartzites, or indurated silicious sandstones; and, as showing that their solidity and hardness are due only to a further continuance of the deposit of silica in the interstices, it has been proved that the purple quartzites are purple only by reason of the original coloration of the grains which have been enclosed between the original grains and the secondary silica. Yellow sandstone is colored also by iron, and I have frequently seen the red sandstone shading of to the yellow without any division whatever. The various shades and tints of sandstone are necessarily due to the coloration of the individual grains.
Most of you will, no doubt, have observed the sort of marbling or grain upon the stone of our old buildings, such as the Town-Hall, which I believe was obtained from quarries occupying the site of the St. James’s Cemetery. This is due to what is called current bedding; that is to say, the grains have been arranged along oblique lines and curves instead of in parallel laminæ. This stone, which is geologically equivalent to the Storeton Stone, and of the same nature, has stood very well. Some of the Storeton Stone, if free from clay galls, although very soft when quarried, becomes hardened by exposure, and will stand the weather much better than a harder and more pretentious material.
The stone of Compton House is in a very good condition, although the mason told me such was the hurry in rebuilding that they could not stop to select the stone, and also that it is placed in all sorts of positions with respect to its quarry bed. Perhaps the circumstances that the stone is not in parallel laminæ may have something to do with its durability, notwithstanding this latter fact.
It would take a long Paper, and several evenings, to exhaust the subject even of our local stones. I may mention, however, that the quarries of Grinshill, between Shrewsbury and Hawkstone, yield a beautiful white sandstone, of a finer grain than Storeton, but of a similar quality.
Most of the public buildings of Shrewsbury are built of it, and I am informed that it was to some extent used in the Exchange buildings. The rocky substratum of a district can be well seen in its ancient buildings, for in old times carriage was so important an item that the old builders could not go far for their stone; hence we see that the old churches of part of Lancashire and most of Cheshire, and a large portion of Shropshire, are of red sandstone. Some of it has stood very well, while some has decayed into shapeless masses. There is a tendency to exfoliate parallel to the exposed or worked surface, in all stones, irrespective of the way of the bed, but more so where the stone is set up on edge, or at right angles, to its quarry bed. It is interesting and peculiar to see in some of the old buildings erected with pebbly sandstone how the white quartz pebbles stand out from the surface like warts. This is due to the greater indestructibility of the quartz pebbles, and the weathering away, or denudation, of the sandstone face.
Before leaving the subject of local sandstones it will be necessary to refer to one quality they have which is of excellent utility as regards the storage of water, but which is decidedly a disadvantage in building stone—that is, their porosity. I have proved by actual experiment that a cubic foot of Runcorn Stone will take up three quarts of water by capillarity, and that it is possible to make a syphon of solid sandstone which will empty a vessel of water into another vessel by capillarity alone.[2] This shows the absolute necessity of damp-proof courses, not only in the main walls of buildings of stone, but even in fence walls, for the continual sucking up of moisture from the earth, and its evaporation at the surface of the stone, make it rapidly decay. I think I could show you this fact in almost any stone building in Liverpool or elsewhere where the stone is in direct connection with the earth. It also shows the necessity of taking care that no stones go through the wall to the interior surface, and of precautions for backing up stone walls with less porous materials, or the introduction of a cavity. If you could suppose such a sandstone wall 40 feet long, 20 feet high, and 1 foot 6 inches thick fully saturated, it would hold almost a ton of water! Of course, it never would be fully saturated, because of the evaporation from the surfaces, but with a southwest aspect, and very wet weather, it might become half saturated. But what does evaporation mean? It means the loss of so much heat and the burning of so much coal to supply its place. From this it will be seen that a pure sandstone wall is a thing to be avoided.
The subject is so wide a one that I have felt compelled to restrict my remarks to local sandstones, but the general principles of structure apply to all sandstones alike.
It is difficult by written description to tell you how to select a good stone, but one essential is that there shall be a good deposition of secondary quartz, as shown by the crystalline sparkling on the freshly fractured surface.
It must also be free from very decided laminations, for these constitute planes of weakness and are often indications of the deposition of varying materials, or the same material in various grades of fineness. It must also not be full of argillaceous and iron-oxide infillings. It should possess a homogeneous texture. The best way to study building stones is to study them in old buildings, for nature has then dissected their weaknesses.