Chapter II: On the Durability of Stone Buildings

On the Choice of a Stone for Building Purposes.

“Everything belonging to the earth, whether in its primitive state, or modified by human hands, is submitted to certain and innumerable laws of destruction, as permanent and universal as those which produce the planetary motions. The operations of nature, when slow, are no less sure; however man may for a time usurp dominion over her, she is certain of recovering her empire. He converts her rocks, her stones, her trees, into forms of palaces, houses, and ships; he employs the metals found in the bosom of the earth as instruments of power, and the sands and clays which constitute its surface as ornaments and resources of luxury; he imprisons air by water, and tortures water by fire to change, to modify, or destroy the natural forms of things. But in some lustrums his works begin to change, and in a few centuries they decay and are in ruins; and his mighty temples, framed, as it were, for divine purposes, and his bridges formed of granite, and ribbed with iron, and his walls for defence, and the splendid monuments by which he has endeavoured to give eternity even to its perishable remains, are gradually destroyed; and these structures which have resisted the waves of the ocean, the tempest of the sky, and the stroke of the lightning, shall yield to the operation of the dews of heaven, of frost, rain, vapour, and imperceptible atmospheric influences; and as the worm devours the lineaments of his mortal beauty, so the lichens and the moss, and the most insignificant plants, shall feed upon his columns and his pyramids, and the most humble and insignificant insect shall undermine and sap the foundations of his colossal works, and make their habitations amongst the ruins of his palaces, and the falling seats of his earthly glory.”[1]

Although it is true that all human works must decay, yet it is a point of great importance to ourselves and our successors whether that decay be slow or speedy. The causes enumerated in the above eloquent passage, though sure, are exceedingly slow in their action, and provided the building materials have been selected with reference as well to their durability as to their beauty, the resulting structure may defy the corroding tooth of time for many ages, and we may thus transmit to a long posterity, lasting memorials of our wisdom and science, as well as of our piety. Modern science has, to a very great extent, enabled the architect and builder to determine beforehand what is the durability of any given stone; and it is with great pleasure that we now notice the extensive inquiry made at the suggestion of Mr. Barry, the architect of the new Houses of Parliament, under the Commission issued by Her Majesty’s Government, to investigate the qualities of stone in various parts of the kingdom, in order to select that which should best ensure perpetuity to this grand national monument. This commission, consisting of Mr. Barry, Sir H. T. De la Beche, Dr. W. Smith, and Mr. C. H. Smith, visited one hundred and five quarries, and examined one hundred and seventy-five edifices; and their collected specimens were then submitted to tests, both mechanical and chemical, by Professors Daniell and Wheatstone, of King’s College, London. In order to leave a permanent record of their labours, the Commissioners published a Report, and deposited in the Museum of Economic Geology, a variety of specimens of the stones which they had collected. From this Report, we select such details as are calculated to serve the purposes of popular instruction. The Commissioners did not consider it necessary to extend their inquiries to granites, porphyries, and other stones of similar character, on account of the enormous expense of converting them to building purposes in decorated edifices, and from a conviction that an equally durable, and in other respects more eligible material, could be obtained for the object in view from among the limestones or sandstones of the kingdom.

The Commissioners soon had striking proofs of the necessity and importance of this inquiry in the lamentable effects of decomposition observable in the greater part of the limestone employed at Oxford; in the magnesian limestones of the Minster, churches, and other public edifices at York; and in the sandstones of which the churches and other public buildings at Derby and Newcastle are constructed; and numerous other examples. The unequal state of preservation of many buildings, often produced by the varied quality of the stone employed in them, although it may have been taken from the same quarry, showed the propriety of a minute examination of the quarries themselves, in order to gain a proper knowledge of the particular beds from whence the different varieties have been obtained. An inspection of quarries was also desirable for the purpose of ascertaining their power of supply, and other important matters; for it frequently happens, that the best stone in quarries is often neglected, or only partially worked, in consequence of the cost of laying bare, and removing those beds with which it may be associated; whence it happens, that the inferior material is in such cases supplied.

Stone buildings decay more rapidly in towns than in the open country, where dense smoke, fogs, and vapours, which act injuriously on buildings, do not exist. There is also another curious cause which contributes to the durability of stone buildings situated in the country. In the course of time, the stone becomes covered with minute lichens, which, though in themselves decomposing agents, act with extreme slowness, and when once firmly established over the entire surface of the stone, seem to exercise a protective influence, by defending the surface from the more violent destructive agents; whereas, in populous smoky towns, these lichens are prevented from forming, and thus the stone is exposed to severer trials than stone of the same kind situated in the country.

As a remarkable illustration of the difference in the degree of durability in the same material, subjected to the effects of the air in town and country, the appearance is noticed of several frusta of columns, and other blocks of stone, that were quarried at the time of the erection of St. Paul’s Cathedral, London, and which are now lying in the Isle of Portland, near the quarries from whence they were obtained. These blocks are invariably found to be covered with lichens, and, although they have been exposed to all the vicissitudes of a marine atmosphere for more than one hundred and fifty years, they still exhibit beneath the lichens their original form, even to the marks of the chisel employed upon them; whilst the stone which was taken from the same quarries, (selected no doubt with equal, if not greater care, than the blocks alluded to,) and placed in the Cathedral itself, is, in those parts which are exposed to the south and south-west winds, found, in some instances, to be fast mouldering away.

Colour is more important in the selection of a building-stone to be situated in a populous and smoky town, than for one to be placed in the open country, where all edifices become covered with lichens; for, although in such towns, those fronts which are not exposed to the prevailing winds and rains, will soon become blackened, the remainder of the building will constantly exhibit a tint depending upon the natural colour of the stone.

The chemical action of the atmosphere produces a change in the entire matter of the limestones, and in the cementing substance of sandstones, according to the amount of surface exposed to it. The particles of the stone first loosened by the action of frost are removed by powerful winds and driving rains. The buildings in this climate were generally found to suffer the greatest amount of decomposition on their south, south-west, and west fronts, arising doubtless from the prevalence of winds and rains from those quarters.

Those buildings which are highly decorated, such as the churches of the Norman and pointed styles of architecture, generally afford a more severe test of the durability of a building-stone, than the more simple and less decorated castles of the fourteenth and fifteenth centuries; because, in the former class of buildings, the stone is worked into more disadvantageous forms than in the latter, as regards exposure to the effects of the weather. Buildings in a state of ruin, from being deprived of their ordinary protection of roofing, glazing of windows, &c., afford an equally severe test of the durability of the stone employed in them.

The durability of various building-stones in particular localities was estimated by examining the condition of the neighbouring buildings constructed of them. Among sandstone buildings was noticed the remains of Ecclestone Abbey, of the thirteenth century, near Barnard Castle, constructed of a stone closely resembling that of the Stenton quarry, in the vicinity, in which the mouldings and other decorations were in excellent condition. The circular keep of Barnard Castle, apparently also built of the same material, is in fine preservation. Tintern Abbey is noticed as a sandstone edifice, that has to a considerable extent resisted decomposition. Some portions of Whitby Abbey are fast yielding to the effects of the atmosphere. The older portions of Ripon Cathedral; Rievaulx Abbey; and the Norman keep of Richmond Castle, in Yorkshire, are all examples of sandstone buildings, in tolerably fair preservation.

Of sandstone edifices in an advanced state of decomposition, are enumerated Durham Cathedral, the churches at Newcastle-upon-Tyne, Carlisle Cathedral, Kirkstall Abbey, and Fountain’s Abbey. The sandstone churches of Derby are also extremely decomposed; and the church of St. Peter, at Shaftsbury, is in such a state of decay, that some portions of the building are only prevented from falling by means of iron ties.

The choir of Southwell Church, of the twelfth century, affords an instance of the durability of a magnesio-calciferous sandstone after long exposure to the influences of the atmosphere. The Norman portions of this church are also constructed of magnesian limestone, similar to that of Bolsover Moor, and which are throughout in a perfect state, the mouldings and carved enrichments being as sharp as when first executed. The following buildings, also of magnesian limestone, are either in perfect preservation, or exhibit only slight traces of decay: the keep of Koningsburgh Castle; the church at Hemingborough, of the fifteenth century; Tickhill Church, of the same date; Huddlestone Hall, of the sixteenth century; Roche Abbey, of the thirteenth century.

The magnesian limestone buildings which were found in a more advanced state of decay, were the churches at York, and a large portion of the Minster, Howden Church, Doncaster Old Church, and buildings in other parts of the county, many of which are so much decomposed, that the mouldings, carvings, &c., are often entirely effaced.

The report speaks in high terms of the preservation of buildings constructed of oolitic and other limestones; such are Byland Abbey, of the twelfth century; Sandysfoot Castle, near Weymouth, constructed of Portland oolite in the time of Henry the Eighth; Bow-and-Arrow Castle, and the neighbouring ruins of a church of the fourteenth century, in the island of Portland.

The oolite in the vicinity of Bath does not seem to wear well.

The excellent condition of the parts which remain of Glastonbury Abbey shows the value of a shelly limestone similar to that of Doulting; whilst the stone employed in Wells Cathedral, apparently of the same kind, and not selected with equal care, is in parts decomposed. In Salisbury Cathedral, built of stone from Chilmark, we have evidence of the general durability of a siliciferous limestone; for, although the west front has somewhat yielded to the effects of the atmosphere, the excellent condition of the building generally is most striking.

The materials employed in the public buildings of Oxford, afford a marked instance both of decomposition and durability; for whilst a shelly oolite, similar to that of Taynton, which is employed in the exposed parts of the more ancient parts of the Cathedral, in Morton College Chapel, &c., is generally in a good state of preservation, a calcareous stone from Heddington, employed in nearly all the colleges, churches, and other public buildings, is in such a deplorable state of decay as, in some instances, to have caused all traces of architectural decoration to disappear, and the ashler itself to be, in many places, deeply disintegrated.

In Spofforth Castle, two materials, a magnesian limestone and a sandstone, have been employed, the former in the decorated parts, and the latter for the ashler, and although both have been equally exposed, the magnesian limestone has remained as perfect in form as when first employed, while the sandstone has suffered considerably from the effects of decomposition. In Chepstow Castle a magnesian limestone is in fine preservation, and a red sandstone rapidly decaying. A similar result was observed in Bristol Cathedral, which afforded a curious instance of the effects of using different materials; for a yellow limestone and a red sandstone have been indiscriminately employed both for the plain and the decorated parts of the building; not only is the appearance unsightly, but the architectural effect of the edifice is also much impaired by the unequal decomposition of the two materials.

After enumerating these and other examples, the Report gives the preference to the limestones, on account of their more general uniformity of tint, their comparatively homogeneous structure, and the facility and economy of their conversion to building purposes; and, of this class, preference is given to those which are most crystalline. Professor Daniell is of opinion that the nearer the magnesian limestones approach to equivalent proportions of carbonate of lime and carbonate of magnesia, the more crystalline and better they are in every respect.

It was considered that this crystalline character, together with durability, as instanced in Southwell Church, &c.; uniformity in structure; facility and economy in conversion; and advantage in colour, were all comprised in the magnesian limestone, or dolomite of Bolsover[2] Moor and its neighbourhood, and was accordingly recommended as the most fit and proper material to be employed in the New Houses of Parliament.[3] This opinion was not arrived at, nor this recommendation made, until after a very extensive series of experiments had been completed by Professors Daniell and Wheatstone upon specimens of the stones of the various quarries visited by the Commissioners. The specimens, as delivered to these gentlemen, were in the form of two-inch cubes. These experiments were of a most comprehensive kind. The composition of the stones was determined by chemical analysis:--their specific gravities; their weights after having been perfectly dried by exposure in heated air for several days; then their weights after having been immersed in water for several days so as to become saturated; the object being to ascertain the absorbent powers of the stones, which was further tested by placing them in water under the exhausted receiver of an air-pump. The stones were also subjected to the process of disintegration, invented by M. Brard, the object of which is to determine, by easy experiments, whether a building-stone will or will not resist the action of frost. Lastly, the cohesive strength of each specimen, or its resistance to pressure, was tested by the weight required to crush it. This weight was furnished by a hydrostatic press, the pump of which was one inch in diameter: one pound at the end of the pump lever produced a pressure on the surface of the cube equal to 2·53 cwt., or to 71·06 lbs. on the square inch. These trials were made with caution; the weight on the lever was successively increased by a single pound; and, in order to ensure a gradual action, a minute was allowed to elapse previous to the application of each additional weight. It was noted for each specimen the pressure at which the stone began to crack, and also the pressure at which it was crushed.

The results of all these experiments (which are stated for each stone) gave a decided preference to the Bolsover magnesian limestone, which was noticed as being remarkable for its peculiarly beautiful crystalline structure, while it was the heaviest and strongest of all the specimens, and absorbed least water. Its composition was 50 per cent. of carbonate of lime, and 40 of carbonate of magnesia; the remaining ten parts consisting chiefly of silica and alumina.

An easy Method of determining whether a Stone will resist the Action of Frost.

In the choice of a stone for building purposes, it is of the utmost importance to be able to determine, by a few prompt and easy experiments, whether the proposed stone is capable of resisting the destructive action of moisture and frost. The means of ascertaining this were difficult and uncertain, until M. Brard, several years ago, communicated his method to the Royal Academy of Sciences at Paris. This learned body having appointed a Committee of their own members to inquire into the merits of M. Brard’s process, and to make a report thereon, the united testimony of engineers, architects, masons, and builders from different parts of France, was received, and proved so favourable as to its merits and simplicity, that the Committee recommended the plan to public notice and general adoption. From their Report we select a few details, which hitherto, we believe, have not appeared in English.

When water is converted into ice an increase in bulk suddenly takes place with such amazing force that it appears to be almost irresistible. This is the force which cracks our water-bottles and ewers; splits asunder the trees of our forests; and destroys some of the stones of our buildings. But the action of frost upon stone is very gradual; it is confined to the surface, and when we see a layer of stone separated from the rock or the building, we see the result of the action of the frost during several successive winters, whereby the fragment is gradually thrust out of its perpendicular position, and at length falls. This natural process is repeated in our buildings: we rarely see squared stones split into large fragments by the action of frost except there be a cavity of some considerable size, in which a quantity of water can be collected. The usual action of the frost is at the surface, which is destroyed by the chipping off of small fragments in consequence of the adhesion of the materials of the stone being partially destroyed.

All stones absorb water in greater or less quantities, and there is no rock that does not contain some humidity. The great difference between stones which is now to be considered is in their power of resisting frost. Stones of the same kind, nay, stones from different parts of the same quarry, are acted upon very differently by frost; for, while one stone soon begins to show the destructive effects of its action, another remains uninjured during many centuries. It will, therefore, be convenient to call those stones, of whatever kind, which withstand the action of frost, _resistant_, and those which yield to its action, _non-resistant_.

M. Brard’s first idea, in order to test these resistant properties in building-stones, was, to saturate the stone with water, and then expose it to cold artificially produced; but this was found to be impracticable on a large scale, and the freezing mixtures and other means of producing cold were liable to act chemically upon the stone, and thus produce other effects than those of cold.

M. Brard was then led to compare water with those numerous solutions of the chemist, which, under certain modes of treatment, crystallize. The expansive force of salts in crystallizing is very great, and he saw no reason why water should not be regarded as a crystalline salt similar in its nature to those saline bodies which effloresce at the surfaces of stones, and in time destroy them and even reduce them to powder.

He therefore tried, in a very large number of experiments, the action upon building-stones of solutions of nitre, of common salt, of Epsom salts, of carbonate and sulphate of soda, of alum and of sulphate of iron, and found that the stones cracked and chipped, and in many cases behaved precisely in the same way as when under the influence of freezing water. In the course of these trials, sulphate of soda (Glauber’s salts) was found to be the most energetic and active, and to be the best exponent of the action of freezing water.

In order, therefore, to determine promptly if a stone be resistant or non-resistant, the following process was adopted. A saturated solution of sulphate of soda was made in cold water; the solution being put into a convenient vessel, the stone was immersed, and the solution boiled during half an hour: the stone was then taken out, and placed in a plate containing a little of the solution. It was then left in a cool apartment, in order to facilitate the efflorescence of the salt with which the stone was now impregnated. At the end of about twenty-four hours the stone was covered with a snowy efflorescence, and the liquid had disappeared either by evaporation or by absorption. The stone was then sprinkled gently with cold water until all the saline particles disappeared from the surface. After this first washing the surfaces of the stone were covered with detached grains, scales, and angular fragments, and the stone being one that was easily attacked by frost, the splitting of the surfaces was very marked. But the experiment was not yet terminated: the efflorescence was allowed to form, and the washing was repeated many times during five or six days, at the end of which time the bad qualities of the stone became fully established. The stone was finally washed in pure water; all the detached parts were collected, and by these the ultimate action of the frost upon the stone was estimated.

The behaviour of various non-resistant stones under this process was remarkable. Some were found to have deteriorated in the course of the third day; others to have entirely fallen to pieces; those of which the power of resistance was somewhat greater, held out till the fifth or sixth day; but few stones, except the hard granites, compact limestones, and white marbles, were able to stand the trial during thirty consecutive days. For all useful purposes, however, eight days suffice to test the resistant qualities of any building-stone.

The explanation of this process is very easy. The boiling solution dilates the stone and penetrates it to a certain depth, nearly in the same way that rain water by long-continued action introduces itself into stones exposed to the severity of our changeable climate. Pure water when frozen occupies a greater bulk than when fluid, and the pores or cellules of the stone not being able to accommodate themselves to the increased bulk of the water, great pressure is exerted between and among them, whereby a portion of the water is driven to the surface, and in doing so rends and detaches small portions of the stone. The same action takes place with the saline solution; it is introduced into the stone in a fluid state, from which passing into the solid it occupies a greater bulk, and a portion of it appears at the surface. The repeated washings have no other object than to allow the salt to exert its greatest amount of destructive action upon the stone. There is a striking analogy between the effect of congealed water and that of the efflorescence of salts, in the disintegration of non-resistant stones; namely, that pure water acts on the stones destructively only in a state of snowy efflorescence, which evidently proceeds from the interior to the exterior like the saline efflorescence; whilst water at the surface of the stones may freeze into hard ice without injuring them, just in the same way as salts, which may crystallize upon stones without exerting any injurious action.

The experience of several engineers, extending as it does over several years, fully proves, of a large variety of stones whose qualities were well known, that the action of M. Brard’s process and that of long-continued frost exactly coincide.

It is not the least interesting part of the inquiry to know that this process may be applied with perfect success to ascertain the solidity and resistant power of bricks, tiles, slates, and even mortar. From a mass of minute detail, we will select a few general results.

During one winter season M. Vicat composed seventy-five varieties of mortar, the difference between any two consisting in the proportion of sand and the method of slaking the lime. In the following June these mortars were exposed to the disintegrating process. Most of them were attacked in twenty-four hours; almost all of them in forty-eight hours; and all except two in three days. This gentleman also found that a mortar made ten years previously, of one hundred parts lime, which had been left exposed to the air, under cover, during a whole year, and then mixed up into a paste with fifty parts of common sand, withstood the trial admirably during seventeen days, while the best stones of the neighbourhood speedily gave way. In this case the solution was saturated while hot, which is so powerful in its effects that stones which have resisted the action of the frost for ages, soon gave way when exposed to it.

M. Vicat calculates that the effect of the sulphate of soda upon a non-resistant stone after the second day of trial equals a force somewhat greater than that exerted by a temperature of about 21° Fahrenheit, on a stone saturated with water.

The action of the process upon bricks proved that, whatever their qualities in other respects, if imperfectly burnt, they are speedily acted on. The sharp edges of the brick, and then the angles, are first rounded, and finally the brick is reduced to powder. Such is precisely the action of frost often repeated. Well-baked bricks, on the contrary, retain their colour, form, and solidity by this process, as well as under the influence of frost. Ancient Roman bricks, tiles, and mortar, and hard well-baked pottery resisted the process perfectly; as did also white statuary marble of the finest quality, while common white marble was soon attacked. In Paris, portions of buildings which had been exposed to the air during twenty years without undergoing the least alteration, were submitted to this ordeal, and the experiment agreed with observation. In one extensive series of experiments on stones from different quarries of France, the action of the salt was continued for seven days, and the results noted down; it was then continued for fourteen days, and the results compared with the preceding ones; which only served to confirm the judgment first given, for those stones which were noted as of bad quality crumbled to dust or split into fragments, while those noted for their good qualities had experienced no sensible alteration.

One of the great advantages of this process is the power it gives to the architect of choosing a hard, durable stone for those parts of the building most exposed to the action of the weather, when the funds are insufficient to admit of the whole building being so constructed. Thus the cornices, the columns, and their capitals, are struck in all directions by rain, and hail, and damp air, and are consequently far more exposed to their destructive action than the flat surface of a wall, which offers but one plane to the air.

In the course of this inquiry a very curious case arose. During the erection of a church in Paris, the architect required a good durable stone for the Corinthian capitals; and many circumstances disposed him to select it from the neighbouring quarry of the Abbaye du Val. But, on seeking the opinion of two brother architects, he was surprised to find their estimations of the stone to be totally at variance, for while one declared that he had employed it with the greatest success, another said that he had seen it yield speedily to the effects of frost. On visiting the quarry it was found that two beds of stone were being worked, an upper and a lower bed; specimens of the stone were taken from each, and on submitting them to a hot saturated solution, it was ascertained almost immediately that the upper layer furnished excellent stone, while the lower one supplied that of which the architect had so much reason to complain. But it is remarkable that the stones from the two beds had precisely the same appearance in grain, colour, and texture; so much so, that when brought into the mason’s yard it was impossible by ordinary tests to distinguish the good from the bad stone.

At the conclusion of the inquiry of the Committee, the Royal Academy of Sciences proved the high estimation in which they held this contribution of science to the useful arts, by directing to be published the following practical directions for repeating the process, for the use of architects, builders, master masons, land proprietors, and all persons engaged in building.

1. The specimens of stone are to be chosen from those parts of the quarry, where from certain observed differences in the colour, grain, and general appearance of the stone, its quality is doubtful.

2. The specimens are to be formed into two-inch cubes, carefully cut, so that the edges may be sharp.

3. Each stone is to be marked or numbered with Indian ink or scratched with a steel point; and corresponding with such mark or number a written account is to be kept as to the situation of the quarry, the exact spot whence the stone was detached, and other notes and information relating to the specimen.

4. Continue to add a quantity of sulphate of soda to rain or distilled water, until it will dissolve no more. You may be quite sure that the solution is saturated, if, after repeatedly stirring it, a little of the salt remains undissolved at the bottom of the vessel an hour or two after it has been put in.

5. This solution may be heated in almost any kind of vessel usually put on the fire, but perhaps an earthen pipkin may be most convenient. When the solution boils, put in the specimens of stone, one by one, so that all may be completely sunk in it.

6. Continue the boiling for thirty minutes. Be careful in observing this direction.

7. Take out the cubes one at a time, and hang them up by threads in such a way that they may touch nothing. Place under each specimen a vessel containing a portion of the liquid in which the stones were boiled, having first strained it to remove all dirt, dust, &c.

8. If the weather be not very damp or cold the surfaces of each stone will, in the course of twenty-four hours, become covered with little white saline needles. Plunge each stone into the vessel below it, so as to wash off these little crystals, and repeat this two or three times a day.

9. If the stone be one that will resist the action of frost, the crystals will abstract nothing from the stone, and there will be found at the bottom of the vessel neither grains, nor scales, nor fragments of stone. Be careful, in dipping the stone, not to displace the vessel.

If, on the contrary, the stone is one that will not resist the action of frost, this will be discovered as soon as the salt appears on the surface, for the salt will chip off little particles of the stone, which will be found in the vessel beneath; the cube will soon lose its sharp edges and angles; and by about the fifth day from the first appearance of the salt, the experiment may be considered at an end.

As soon as the salt begins to appear at the surface its deposit is assisted by dipping the stone five or six times a day into the solution.

10. In order to compare the resisting powers of two stones which are acted upon by the frost in different degrees, all that is necessary is, to collect all the fragments detached from the six faces of the cube, dry them and weigh them, and the greatest weight will indicate the stone of least resistance to the frost. Thus, if a cube of twenty-four inches of surface loses 180 grains, and a similar cube only 90 grains, the latter is evidently better adapted than the former to the purposes of building.

FOOTNOTES:

[1] SIR HUMPHRY DAVY.

[2] Bolsover is a small market town in Derbyshire, on the borders of the county of Nottingham, and about 145 miles from London.

[3] The various quarries visited by the commissioners are noticed in the fullest and fairest manner. They have stated for each quarry its name and situation; the names and addresses of the freeholder, of his agent, and of the quarryman; the name of the stone; its composition; colour; weight per cubic foot; entire depth of workable stone; description of the beds; size of blocks that can be procured; prices, per cubic foot, of block stone at the quarry; description and cost of carriage to London; cost, per cubic foot, of the stone delivered in London; cost, per foot of surface, of plain rubbed work, as compared with Portland stone; and, finally, where known or reported to have been employed in building.