For working compressive strength of stone masonry, see Appendix I. Transverse Strength. The transverse strength is not necessarily related to the crushing strength. Following are the ranges shown by several classes of stone:

Elasticity.-Buckley gives the following figures showing modulus of elasticity in Wisconsin

Shearing Strength-Shearing is a form of stress likely to be brought to bear on stone in some parts of buildings. Three granites tested showed a shearing strength of from 1742 to 2872 lb. per sq. in.; three sandstones ranged from 992 to 1383; and two marbles gave 1163 to 1554.

Frost Resistance.—Alternate heating and cooling of a stone causes expansion and contraction, which may be detrimental to it, but if the stone is dry the injury is insignificant. With water present in the pores, the effects may be quite different. When water freezes it expands, and if this water is imprisoned in the pores of the stone, it may exert sufficient internal pressure to split it, and so since the amount of water which may be present in a stone is related to the pore space, its resistance to frost is closely connected with that of porosity.

The frost resistance of a stone depends on: (1) whether or not the pores are full of water at the time freezing occurs; (2) the shape and size of the pores, large and straight pores either allowing the water to drain off rapidly before it does any damage, or else permitting the ice to force its way outward thus relieving internal pressure; and (3) the amount of pore space, for the higher the percentage, provided the pores are of equal size and the degree of saturation equal, the greater the damage from freezing.

Pores may be of two kinds, capillary and sub-capillary, the latter remaining filled under ordinary draining. Under normal conditions, only a small proportion of the sub-capillary pores become filled with water and the worst possible condition would be represented by the complete filling of these. So a stone with a large proportion of fine pores is more liable to be injured by freezing. Therefore, the ratio between fine pore space and total pore space, gives a factor in judging the ability of a stone to withstand frost. Hirschwald obtains this factor as follows:

A dried and weighed test-cube is immersed in water for from 1 to 2 hr. if the stone is to be used above ground, and for from 2 to 30 days if the stone is to be used below the ground. The increased weight of cube represents the fine pore space that would be filled under worst possible natural conditions. Next the cube is completely saturated by immersion under vacuum or strong atmospheric pressure, and weighed again. The quotient obtained by dividing the increase representing the fine pores by the increase representing the total pores gives the proportion of fine pore space to total pore space and is termed the saturation coefficient. If the quotient is greater than 0.9, there is danger of the stone being injured by frost, but if less than 0.9 no injury can result.

Parks in carrying out the above tests on a series of Ontario stones for work above ground, found that the saturation coefficient for granites and gneiss es ranged from 0.67 to 0.8; seven marbles ranged from 0.44 to 0.94, the finer grained types giving higher results than coarse grained ones; sandstones ranged from 0.21 to 0.57, while the limestones varied from 0.11 to 0.91.

The usual but unsatisfactory method of testing frost resistance consists in thoroughly drying a cube of stone and then weighing, after which it is soaked in water for 36 hr. It is then subjected to 40 alternate freezings and thawings. Following this the stone is dried and reweighed. Loss in weight indicates particles chipped off by freezing.

Fire Resistance.-Building stones often suffer serious injury when exposed to fire, or the combined effects of fire and water. Stone expands when heated and contracts when cooled, but the amount for a bar 1 ft. long, heated 1 deg., is exceedingly small, granite for example expanding 0.000004825 in. per ft. for each degree. When subjected to fire, a stone is rapidly heated and expanded, and if doused with water undergoes equally rapid cooling and contraction. Moreover, stones are poor conductors of heat, hence the exterior of a large block may be quite hot, while the interior is still cool, thus setting up stresses which disrupt the stone. Few stones have good fire resistance as witnessed by their spalling off during conflagrations. However, some stand up better than others. Rocks of close fabric, interlocked grains, and simple mineral composition seem to show the best resistance. Tests by McCourt indicated that most stones were fairly resistant up to 550 deg. C. At 850 deg. C. all were more or less injured; granites and gneisses spalled and cracked; sandstones parted along the bedding planes, a few developing cross fractures; limestones were little injured up to temperature of calcination but after that failed badly; marbles developed cracks before the calcination temperature was reached.

In testing for fire resistance, a cube of stone of not less than 3 or 4-in. size is employed, smaller sizes giving unreliable results as the stone gets heated through too readily. Pairs of cubes are heated to 550 and 850 deg. C., respectively, one of each pair being allowed to cool slowly, the other cooled rapidly by being plunged into cold A fifth cube is exposed to a large flame blast for 5 min., allowed to cool in air for 1 min., and again blasted, this alternation being repeated until the stone cracks. A sixth cube is alternately exposed to the action of the blast and a stream of cold water. All damage to the cubes is noted.

water.

Abrasive Resistance. This property depends on the state of aggregation of the mineral particles and in part on their individual hardness. Different stones wear very differently, and one of uneven hardness may wear in very irregular manner. The use of stones in steps or even floors of public buildings where there is much passing, serves well to bring out their resistance to abrasion.

Abrasion may also be caused by wind blown sand and dust and the effects of this will sometimes smooth or polish rocks as hard as quartzite.

It is not uncommon to find uneven marble floors due to the fact that tiles of uneven hardness set side by side have worn down very unevenly.

A common method of testing abrasive resistance consists in laying a slab of this stone to be tested on a grinding table, weighting it down, and applying emery or some other abrasive at a given rate while the table revolves a certain number of times. The stone is weighed before and after the test and the loss in weight noted. An objection to this method is the difficulty of feeding the abrasive uniformly under the test piece.

A second suggested method consists in forcing sand through a 6-cm. opening under a dry steam pressure of 3 atmospheres for 2 min. The stone to be tested is held immediately over the opening. This test is supposed to determine not only the extent to which the stone will be abraded under the given conditions, but also brings out irregularities in hardness.

Specific Gravity and Weight per Cubic Foot.-The apparent specific gravity of a rock may differ from the specific gravity of its component minerals, the former being influenced by the porosity. A rock of high porosity will have a low apparent specific gravity.

Bowles gives the following figures illustrative of the above:-Friable sandstone: specific gravity, 1.825; weight per cubic foot, 113.1 lb.; ratio of absorption, 1.8. Quartzite: specific gravity, 2.729; weight per cubic foot, 170.6 lb.; ratio of absorption, 1.566.

Softening Effect of Water.-The cementing material of some stones, such as sedimentary ones, may be softened by contact with water. The degree to which the stone is affected is

taken as an index of its durability. By determining the tensile strength of a dry piece and one that has been soaked for 28 days, and dividing the latter by the former we obtain the softening coefficient.

Corrosion by Gases.—Both oxygen and carbon dioxide when brought in contact with stone through the medium of moisture, may cause corrosion. Oxygen will cause the change of pyrite to limonite, or the rusting of other iron minerals. Carbon dioxide causes slow superficial disintegration at least by solution of carbonate compounds. Sulphurous fumes may be more injurious than those of carbon dioxide. Tests along these lines, carried on for several weeks or months, give measurable results.

The test for corrosion may be carried out as follows: Cubes of approximately 1-in. size may be used, dried at 110 deg. C., carefully weighed, and the exact superficial area determined. They are then suspended by threads in a bottle of distilled water into which a stream of carbonic acid gas is conducted. The water is renewed every few days and the treatment continued for 4 weeks. At the end of this time the specimens are removed, washed in distilled water, carefully rubbed with the finger tips to remove loose particles, thoroughly dried, and weighed. The loss in weight denotes amount of damage caused by the carbonic acid gas and may be expressed in grains per square inch of surface exposed.

Microscopic Examination of Stone.-Microscopic examination as an aid to the study of building stone, has received considerable emphasis in recent years. Such an examination should be made by one familiar with the subject and may give valuable information regarding structural defects likely to cause trouble, the cause of differences in workability of two stones, the presence of injurious minerals not easily seen with the naked eye, etc.

Sonorousness. This test is specially applied to marbles and slates. When a good slate or sound dense piece of marble in form of a slab is suspended, it gives a clear sound when struck with a hard object. Mica slates are usually more sonorous than clay slates. Solid massive marbles are more sonorous than brecciated ones.

Special Tests for Slate.-All of the tests previously referred to may be applied to slate, but certain ones may be applied because of certain special uses to which it is put. These are as follows:

Cleavability. This is to determine the ease and smoothness with which the slate cleaves. It should be determined by a good workman using a thin chisel with a 2-in. edge.

Character of Cleavage Surface. In general, the smoother the surface the better, as it gives less chance for lodgment of injurious materials. Its smoothness may be examined with a hand lens. A good slate also, when scaled along cleavage, should show thin chips with translucent edges. Most good slates show little or no observable texture to the naked eye.

Presence of Lime Carbonate.-This is determined roughly by treating the slate with dilute hydrochloric acid. A slate with high lime carbonate content is generally less durable.

Presence of Magnetite.-To roughly determine this an inch cube may be pulverized, and tested with a magnet. For electrical purposes slate with magnetite is undesirable.

Toughness or Elasticity.-This is tested by measuring the deflection when a slate is placed on supports 22 in. apart, and pressure applied from above.

Corrodibility.-The resistance to corrosion is determined by immersing a weighed piece of slate in a solution consisting of 98 parts water, 1 part sulphuric acid, and 1 part hydrochloric acid. After soaking 40 hr., the stone is dried and weighed, the loss in weight being noted.

Special Tests for Marble.-In addition to the usual tests that may be applied to all building stones, the following are specially used for marbles:

Porosity. A sawed block 2 X 1 X 1 in. is thoroughly dried out and then immersed for 48 hr. in a concentrated 4% alcoholic solution of nigrosine, a deep blue dye soluble in alcohol. After drying for half an hour the blocks are split with hammer and chisel, and the degree of porosity is indicated by the extent to which the color has penetrated the blocks.

This test is important because of the frequent combination of metals and marbles on exposed faces. oxidation of the metal yields coloring compounds which may be absorbed by the marble.

The

Translucence.-The marble is cut into thin slabs and the degree to which it transmits light may be determined by ordinary photometric methods. Marbles show great differences in their light transmitting capacity, and this effects their translucence. Few marbles have been tested in this manner. The best Pentellic marble allowed light to penetrate 0.59 in., Parian marble, 1.37 in., Carrara statuary marble, 1.18 to 1.57 in.

Statuary Test.-Marble for statuary purposes should be inspected on a dull day with a good light, the surface examined being wet. It should show uniform texture, fine grain, and freedom from veins and discoloration.

Durability of Stone.-This is a question of great practical importance. No stones are of eternal lasting power but some withstand the weathering agents for several centuries while others show signs of decay in a few years or even in a few months. The factors governing durability are: (1) physical and mineralogical characters of the stone; (2) climatic conditions; and (3) location in building. Much valuable information can be obtained by observing stones in buildings long exposed to weather or the weathered surface in quarries.

Julien, as a result of observation on buildings in New York City, gives the life of different stones in that climate as follows:

A stone for interior work does not require the weather resistance of one for outside work. Stone for exterior use meets different conditions whether used above or below ground, or even whether exposed in a vertical surface where water drains off rapidly, or in a horizontal surface where it can accumulate.

The changes produced in a building stone are those incident to ordinary weathering and may be of either physical or chemical nature. Among these, frost action is as a rule the most destructive, searching out the most minute cracks and causing the chipping or flaking off of pieces of rock. The Connecticut brownstone so extensively used in former years in the eastern cities and usually set in the building on edge, often shows serious injury from frost. Warmth and humidity are also potent agents of weathering.

Certain structural irregularities, like grains or lumps of pyrite, veins of calcite, fossil shells, and even chert nodules may hasten the decay of a stone.

In the quarry the rock adjacent to weathered joints is often sufficiently altered to require rejection.

Acid gases in the atmosphere, coming in contact with limestones or marbles, or sandstones containing calcareous cement, also work for the slow destruction of the stone. In the latter case, sulphuric acid gases attacking carbonates may form soluble sulphates, which are brought to the surface as the wet stone dries out. There they may form a white scum on the surface or if the salts crystallize in the pores of the stone just below the surface, a scaling off of the stone is likely to follow.

18. Styles of Dressing Stone.—Rubble is stone of all shapes and sizes that is laid up with little or no regularity. Walls laid up this way are known as rubble work. Coursing stone is a term applied when the wall is laid up in tiers or courses. The stones may or may not be cut to equal length to resemble brick work. Random coursing refers to walls built up of rectangular and bedded blocks of various sizes. Dimension stone properly applies to stone cut to size.

The recognized methods of surfacing are: (1) rock face-natural broken surface of the stone; (2) pointed face-surface dressed comparatively flat by means of the point; (3) hammered face-surface made plane by hammering with patent hammers of different kinds; (4) ribbed or tooth chiseled-surface obtained by using a wide flat-toothed chisel, or more often produced by machinery, a common type of finish for many soft stones; (5) sand finished— produced by rubbing a surface smooth with sand, often applied to marble for exterior work.

19. Dressing Machines-Gang Saw.-Used for cutting large blocks up into slabs. Consists of an upright rectangular frame with a large post of wood or steel at each corner. Suspended

from this and free to swing between the posts, is a horizontal steel sash which can be raised or lowered. The sash is given a sawing motion by means of a pitman, which in turn is operated by a belt-driven crank attached to a fly wheel. The saws, which are bands of soft steel 3 in. wide and 3% in. thick, are stretched between the head pieces of the sash, and held in position by keys. The spacing of the saws determines the thickness of the slabs cut. During operation a continuous supply of sand and water is fed over the stone and it is the sharp sand which cuts the stone. The overflow is caught in a hopper below the block, and used over again. The above type is generally used for marble and limestone. For granite and hard sandstone a modified form of structure is used, the saw blades are heavier and notched on the edge, and chilled shot or crushed steel is the abrasive.

At one Vermont marble mill the standard saws will cut a block 10 ft. long, 6 ft. wide, and 6 ft. high. At another mill, saws working on blocks of marble 6 ft. long, sink 1 in. per hour. Rubbing Bed. This consists essentially of a revolving steel plate or table from 4 to 14 ft. in diameter, and driven at a rate of 44 to 48 revolutions per minute. A wooden box surrounding the plate prevents scattering of sand and water. The stone is placed face down on the rubbing bed and weighted. A cross bar above the table also holds it stationery. Sand and water are supplied during the operation.

Planing Machines.—In these the stone is moved to and fro on a horizontal bed, while it is subjected to the cutting of grooves, channels, or cornices.

Lathes. These are used for turning columns or other round work. The cutting is usually done with a fixed chisel, but in granite work, the chisel is replaced by a steel disc set obliquely against the stone and rotating with it.

Gritting and Polishing Machines.-These give the stone further treatment after the rubbing bed. The machines vary in weight and rigidity according to class of work. Marble gritters and polishers differ in character of abrasive and speed of rotation. With granite, the head or polishing surface is quite different from that used for marble.

The machine consists of a horizontally rotating disc to which different types of head can be attached. The upright spindle and disc are rotated at the end of a jointed adjustable arm so that the polishing surface may be moved over all parts of the stone. For marble gritting the heads are 12 in. diameter, with blocks of abrasives attached in a radial manner on the under side. Finer grades are used as polishing proceeds. Scotch hone is used for final operation, operating at 200 r.p.m. Polishers or buffers have felt heads about 20 in. in diameter, operating at 400 r.p.m. Putty powder produces the gloss.

Diamond Saw. This is used for making single cuts. There are several types but all consist essentially of a steel disc in the margin of which a number of carbons are mounted.

Carborundum Machines.-Carborundum wheels are extensively used in marble finishing shops for curved work, moldings, cornices, balusters, etc. The carborundum wheel is first set in a lathe, and with a steel tool cut to shape of negative of pattern desired. The wheel is then placed on a shaft and marble block on the machine bed travels beneath it. Balusters are turned out quicker this way than on lathes. A Vermont company completes in 1 hr. a baluster 31⁄2 ft. long and 6 in. greatest diameter. Carborundum saws are also used in fluting marble columns.

20. Properties, Distribution, and Uses of the Most Important Building Stones. 20a. Igneous Rocks.-Many kinds are used in structural work. Among the harder denser ones, the granites find greatest favor because of abundance, lighter color, and structural features in the quarry. Syenites and diorites are rare and in little demand. They possess no advantage over granite. Gabbros are too dark to suit most architects but occasionally are of value for decorative purposes. Diabase is hard and not usually obtainable in large blocks, but has been used for paving.

The volcanic rocks, including many lavas and tuffs, are abundant in the far west and Mexico. They are often porous and soft and adapted for work in a dry climate above ground. The granites deserve special mention.

Granite. This term is sometimes rather loosely used and may even include gneiss. Diabase and gabbro are sometimes called black granite. Only granites proper are here considered.