"'It is a fact that with particles of different sizes as against uniform size the densest mixture can be obtained. This is so evident as to require no proof.' It follows that the least density and hence the largest percentage of voids occur when the grains are all of the same size, and it is shown that the most voids occur in a ma.s.s of large particles. The least voids occur when the voids between the large particles are filled with smaller particles and when these smaller voids between the smaller particles are in turn filled with still finer particles. In other words--particles with three determining sizes will fill up a given s.p.a.ce more completely than particles of two determining sizes and very much more completely than particles of one size.

=Elasticity and Strength.= "The paint coating here again is governed by many of the laws which govern the similar material, i.e., concrete. We find, by again referring to Taylor & Thompson, on Concrete, page 275, that tests at the Watertown a.r.s.enal on concrete convinced the investigators that the ultimate strength of a concrete is identical with the shearing strength of particles of stone making up the aggregate.

"This means that in its ultimate form the good concrete will crack or shear through the broken rock contained therein, and resistance to shearing is directly proportionate to the strength of the broken rock chosen for the mixture. The film of semi-liquid linseed oil when fresh is extremely weak, but as it hardens, its characteristics and physical properties will obviously be those qualities which are a composite of the qualities of the solid particles and of the semi-solid linolein incorporated together in the paint coating. These physical properties of the suspended and incorporated pigments profoundly modify the film in this respect.

"The dried vehicle, linoxin, is notable for its elasticity, and it is weak in crushing and tensile strength, and in hardness or resistance to surface wear. The fact that it is a semi-solid furnishes an opportunity to modify and improve those characteristics of a solid in which it is deficient. The semi-solid, rubber-like linoxin between the coa.r.s.er particles of the pigment obviously uses these coa.r.s.er particles as supporting points. The medium sized particles of the second group of alteration products serve the same purpose as the broken rock in concrete. The coa.r.s.er particles absolutely do not, and can not, serve the purpose of stiffening or of reinforcing or modifying the consistency and qualities of the semi-solid linoxin, for a number of reasons, one of which may be mentioned, namely, that particles of the first, or coa.r.s.e, cla.s.s have a determining size which is a large fraction--a heavy percentage--of the total thickness of coating, and are in some instances thicker in diameter than the thickness of an oil coating not reinforced with the fine or fire group.

"We must think of the coa.r.s.er particles as piers. The mixture of linoxin with the other two groups of particles in the s.p.a.ces between these coa.r.s.er particles, or piers, is the true paint body and consists of flat reinforced arches which have the extra support of falsework, in the shape of the structural material on which the coating rests. Asbestine pulp, a natural product and one of our most important natural reinforcing pigments, serves not only in the coa.r.s.e group as supporting particles for the linoxin arch, but also because of its peculiar properties serves the more important purposes of reinforcement. It retains, no matter how finely ground, its peculiar needle-like, or rod-like, form of particles, and obviously serves the purpose of reinforcing the flat arch of linoxin, exactly as iron bars or iron netting serve in reinforced concrete arches. The medium sized particles of the second group of pigments produced by chemical alteration or precipitation, serve the purpose of the broken rock in concrete, and together with the coa.r.s.er supporting particles and the finest reinforcing particles, give minimum voids and a maximum imperviousness to agencies of internal decay.

"It goes without saying that the pigments of any one group contain particles of dimensions which fall into the other two groups, but no one pigment supplies the correct proportion of each of the three required dimensions, and each pigment has so large a percentage of approximate dimensions as to bar it from exclusive use in the other two groups.

Given similar h.o.m.ogeneous coatings under identical conditions, we recognize the law that elasticity will vary directly with thickness.

Direct deduction from this law teaches us that of two paint coatings equal in wear, in strength, opaqueness, and in all other qualities except thickness, we should choose the thinner coating. Therefore if we have two paint coatings fulfilling every requirement, the first compounded with pigments giving a thicker coating and the second with pigments yielding a thinner coating, we must choose the second formula and obtain the thinner coating.

=Adhesive Power.= "The adhesion of the linoxin to the coa.r.s.e group of particles and to the underlying material is vital to the life of the paint coating. If the coating parts from the surface beneath, we have scaling or peeling. It is universally admitted that this will result from use of zinc oxide as the sole pigment. We have only to conceive of our flat arch of reinforced linoxin and leave out our points of support, to realize that this is the inevitable result if the coating be subject to extreme exposure, although good results may be obtained from zinc oxide used alone, as, for instance, in interior house painting where extreme changes of temperature and exposure are avoided.

"Three major lines of force hold our linoxin in place--adhesion toward the underneath surface, adhesion to the coa.r.s.e particles, and cohesion within the linoxin itself. These lines must be represented by a flat arch of linoxin with a downward pointing magnet therefrom, to represent adhesion to the surface. Magnets on each side of the arch pointing toward the supporting coa.r.s.e particles, and two magnets within the arch and pointing toward each other, or to the centre of the arch, these latter to represent the force of cohesion."

CHAPTER VI

THE SCOPE OF PRACTICAL PAINT TESTS

=The Pigment Contention.= During the year 1906 officials of the North Dakota Agricultural Experiment Station examined a number of paints on sale in the northwestern States. The presence of large quant.i.ties of inert pigments as well as water, in some of these paints, prompted agitation for State laws requiring the formula-labeling of paints.

Certain paints made of white opaque pigments such as white lead and zinc oxide were exempted from the statute. The white opaque pigments used in these paints were believed by certain manufacturers as well as by many prominent paint authorities of high standing to be benefited in their wearing value by the addition of small percentages of inert crystalline pigments, such as barytes, silica, China clay, etc. Laboratory experiments had already determined that these inert crystalline pigments had a certain definite action in increasing the life of paints, but it had become evident that they should be used with discretion, in moderation, and with a proper understanding of their limitations, if the best results were to be obtained. The addition of very large quant.i.ties of such pigments was not indulged in by discriminating manufacturers, but the exact percentage to use was a matter of great doubt, even to the most experienced. In order to determine just what percentage of crystalline pigments, admixed with white opaque paint pigments, would give the best service and results, it seemed imperative that practical paint tests should be made. A series of paint tests on commercial brands of paint had already been started at the Fargo Agricultural College, and, at the suggestion of the Paint Manufacturers' a.s.sociation of the United States, another series of practical paint tests were inst.i.tuted, and carried out under the supervision of Dr. E. F. Ladd, Director of the North Dakota Experiment Station.

=Test Fences to Solve the Problem.= It was apparent that the pigment question could be solved only through field tests made on a comprehensive basis and placed under the control of scientific and technical societies of renown, so that they might be fair and unbiased from every standpoint. In order to secure a comparison of the wearing of different paint formulas in various sections of the country and under differing climatic conditions, another series of tests was started in the East soon after the North Dakota tests had been started.

Simultaneously fences were erected at Atlantic City, N. J., and Pittsburg, Pa. The site of the Atlantic City fence is a strip of land running due north from Atlantic and Savannah Avenues and within a short distance from the Atlantic Ocean, the exposure being a severe one. The site of the Pittsburg fence is back of the athletic field of the Carnegie Technical Schools, the fence running east and west and being exposed to the heavily charged sooty atmosphere coming from the many industrial plants near by.

=Construction of Framework of Fences.= At these two locations framework fences were built, upon which were placed a series of painted panels.

Heavy yellow pine posts six inches square were set in the ground about six feet apart and to the depth of about four feet, upon a concrete base. The posts were solidly tamped and then braced at the top with supplementary studding braces two inches thick. Connecting the posts was a line of studding six inches by two inches, forming a solid framework, the bottom of which was approximately fifteen inches from the ground.

The bottoms and tops of the fences were protected by heavy boards two inches thick, so that the moisture and rain might be prevented from working itself up into the wood. The whole fence was sheathed with twelve-inch planed white pine, thus forming a solid background for the test panels.

=Lumber for Panels.= The lumber for the test panels was most carefully selected, being of three grades--white pine, yellow pine, and cypress. A large amount of each grade of lumber was secured, and after the best portion had been made up into panels, the panels were inspected by an expert lumber cla.s.ser; nearly 40% being rejected on account of the presence of knots or sappy places which appeared upon the surface. Each of the panels finally pa.s.sed upon as suitable for the test was branded with a hot iron with consecutive numbers running from 1 to 186. The grade of wood used for each panel was indicated by an abbreviated mark--W for white pine, C for cypress, and Y for yellow pine. In order that a record of each panel might be kept on file, previous to the application of paint to the panels, a complete series of photographs was taken of the panels in sets of four. This work seemed advisable so that the future failure of paint on any one panel, which might be thought due to faulty wood, could be either verified or refuted by a reference to the series of photographs made of the bare panels.

[Ill.u.s.tration: View of Atlantic City Test Fence]

=Construction of Panels.= The panels were constructed of Dutch weather boarding, tongued and grooved together in strips of three pieces and capped at the top with a weather strip, forming a finished surface three feet long and fifteen and a half inches high. They were firmly braced together at their backs and nailed in such a manner that no portion of the nails would appear on the surface of the panel, thus preventing the staining of the panel from rust. The construction of the framework of the fences at Atlantic City and Pittsburg was of such a nature that they would each accommodate 560 panels of this type.

=Starting of Tests.= On account of the lateness of the season, it was found necessary to do the painting of the tests within a building, so that each formula might be subjected to fair and equal conditions of application, thus excluding the blowing of dust or rain upon the painted surfaces, which would have taken place had the panels been painted upon the fence. The painting of the panels began in January, 1908, the temperature within the buildings in which the work was done averaging 50 degrees Fahrenheit throughout the work.

It was decided to test each formula in three colors, in duplicate, and on each grade of wood, exposing the duplicates on either side of the fence. Thus for one paint formula there were required 18 panels, or 6 painted in each color and on 3 grades of wood.

=Paints for Tests.= The mixed paints received for the tests were in quart cans, having been especially prepared from the formulas submitted to manufacturers by the technical committee in charge of the work. They were properly labeled with their number and color, in each case. The formulas decided upon for the test are described later. The various white leads and other single pigment paints which were used were received in kegs weighing 12-1/2 pounds each, having been bought in the open market and then given a formula number. The formulas of the paints designed for both the Atlantic City and Pittsburg tests, as well as the numbers of the panels upon which the paints were applied, are shown on pages 131-133-145. The a.n.a.lysis of one of the combination paints applied is herewith given, to show the correct method of stating the composition of a paint.

FORMULA NO. 20, ATLANTIC CITY TEST FENCE

Percentage Composition

===================+=======+=======+=======+======= |Pigment|Vehicle| Total | -------------------+-------+-------+-------+------- Corroded white lead| 67.01 | -- | 42.84 | Zinc oxide | 19.89 | -- | 12.71 | Asbestine | 3.86 | -- | 2.47 | Calcium carbonate | 9.24 | -- | 5.91 | Raw oil | -- | 94.30 | 34.02 | j.a.pan drier | -- | 3.89 | 1.40 | Turpentine | -- | 1.81 | 0.65 | -------------------+-------+-------+-------+------- |100.00 |100.00 |100.00 | ===================+=======+=======+=======+=======

=Brushes.= Heavy 7-O round bristle brushes were used for the priming coat so that the paint might be well worked into the wood, while for the second and third coats three-inch chisel edge brushes were used. These brushes were, of course, washed several times with turpentine after painting each panel, so that pigments from one paint could not be carried over into a paint containing other pigments.

[Ill.u.s.tration: Cypress Panels]

=Sh.e.l.lacking Panels.= The sh.e.l.lacking of any bad places of minor nature which may have been present on the surfaces of some of the panels, was done with the highest grade orange sh.e.l.lac. It was thought advisable to determine whether sh.e.l.lacking over the priming coat of paint or on the bare wood previous to the application of the priming coat, was the better method. Panels Nos. 1 to 8 in each test were therefore sh.e.l.lacked over the priming coat, while on all other panels the sh.e.l.lacking was done directly on the bare wood previous to the application of the priming coat of paint.

=Application of Paints.= In order to determine just how much paint was applied to each panel and to reckon the spreading rate therefrom, careful weighings were made during the application of every paint. This was carried out by placing a quart can of paint as received, upon a laboratory balance, the gross weight being taken and recorded. The can was shaken and its contents transferred to a quart-size enameled cup where with the aid of a paddle it was broken up into a mixture of even consistency. A portion of this paint was then transferred to two small sample cans carefully numbered with the formula number, for future reference and a.n.a.lysis. The reduction of the paint was then made. The brush used on the priming coat was placed with the pot and the paint on the balance and the weight taken by the official weigher. The pot was then given to the painter who applied the priming coat to one panel. The brush, pot, and paint were then handed back to the official weigher and the difference in weight recorded. From these data could be reckoned the spreading rate of the formula applied.

The drying of the panels was noted every few hours and observations made to determine whether the paints were penetrating properly into the surface of the wood. A period of eight days was allowed between each coat in order that thoroughly hard setting might take place.

During the application of the second coat of paint to the panels, fresh cans of paint were used in every case so that fresh reductions could be made of the proper consistency. Full data were also recorded on the ease of application, working, and nature of drying shown, as well as appearance presented by each paint after each coat had been applied. New packages of paint were also used for the third coat, and, as a rule, the paint was applied without reduction or with full oil reduction, turpentine being eliminated in nearly every case for the third coat work.

=Reductions.= The single pigment paints, such as white leads, were reduced by the so-called ounce system, each ounce of oil added to 12-1/2 ounces of paste pigment representing one gallon of vehicle to one hundred pounds of lead. A complete report of the reductions, spreading rates, etc., used in the tests would take up three or four hundred pages of printed matter. The reductions shown on the following formulas are, however, fairly representative of the reductions used on the combination and single pigment paints.

REDUCTIONS ON FORMULA NO. 2

_White and Yellow_

1st Coat Condition when opened--good. Consistency when broken up--heavy. Reduction recommended by manufacturer--none. Reduction used--3 pints raw oil, 1 pint turps, 1 gallon paint. Consistency after reducing--good, stiff. Working--fair. Drying--fair on pines; cypress--poor. Penetration, pines--good; cypress--poor.

2nd Coat Consistency when broken up--heavy. Reduction used--1-1/2 pints turpentine, 1 pint boiled oil. Consistency after reducing--good.

Working--good. Hiding--medium. Drying on pines--good; cypress--poor.

One-half pint j.a.pan added to gallon of paint. Penetration--fair.

3rd Coat Reduction used--1-1/2 pints oil, 1/2 pint turpentine.

_Reductions for Lead Pastes_ Calculated on 100 lb. keg.

Formulas Nos. 37-38. (Corroded White Lead.)

1st Coat 6-1/2 gallons oil, 1/2 gallon turpentine, 1 pint turpentine j.a.pan.

2nd Coat 3-1/2 gallons oil, 1 gallon turpentine, 1 pint j.a.pan.

3rd Coat 3 gallons oil, 1 pint turpentine, 1/2 pint j.a.pan.

=Hiding Power of Paints.= When the priming coat had thoroughly dried on each panel, the painter carefully stencilled a black Geneva cross over the priming coat with lampblack in oil. The object of this black cross was to make a determination of the comparative opacity or hiding power of the different paints applied. It is well known that various pigments when ground in oil differ in their hiding power in direct proportion to the difference in the refractive indices of the pigments and oils used, those containing high percentages of pigments such as white lead and zinc oxide being superior in hiding power. After the second and third coat of paint had been applied to each panel, there was evident a remarkable difference in the hiding power, as the black cross showed through in some cases quite clearly, while in other cases it was almost completely hidden. The hiding power of a paint is one of the properties which the master painter looks upon as most essential, but it should, of course, be accompanied in a satisfactory paint by good spreading power and longevity.

=Actinic Light Tests.= After the drying of all the paints, it was decided that it would be of extreme interest to conduct a test on the resistance of certain paints to actinic light. It is well known that the ultraviolet or chemical rays of the sun are most energetic in causing chemical reactions that result in the early decay of certain types of paint. It was thought that the disintegrating effect of these rays, as well as their effect in the bleaching out of colors, might be prevented by placing upon certain panels small orange colored gla.s.s slides which would prevent the pa.s.sing of these rays to the painted surface. The slides used were five inches long and three inches wide and were placed upon the middle board of certain panels, with picture framing, putty, and galvanized iron tacks. The preservation of the underlying surface from the sun's rays would, it was thought, prevent the deterioration of the paint, and at the same time preserve its original color so that it might be compared to the color of the exposed portion at the time of inspection.

=Supervision of Tests.= The Atlantic City tests were under the constant supervision of Committee E of the American Society for Testing Materials, this committee having accepted the inspection of the fence. A representative was constantly present throughout the work in order to see that each formula received fair treatment. The actual painting work was under the supervision of the writer, together with a master painter representing George Butler who was chosen by the Master Painters'

a.s.sociation of Philadelphia as the official painter of the Atlantic City test fence. Mr. J. B. Campbell of Chicago also acted as an official of the Paint Manufacturers' a.s.sociation in the application of the formulas to both the Atlantic City and Pittsburg fences.

At Pittsburg the fence was placed directly under the supervision and control of the Carnegie Technical Schools, who chose for the fence work a committee of their technical force. Drs. James and Schaeffer of this inst.i.tution were present throughout most of the work and were constantly represented during the test. The Pittsburg Master Painters' a.s.sociation appointed a committee consisting of Messrs. Dewar, Rapp, and Cluley, for the actual painting work, and they were represented with the writer throughout the tests.

Great interest was exhibited in the work by the committees in charge, and the skill of the practical painters, combined with the care of the inspectors, made the treatment of each formula fair and satisfactory.