A LABORATORY STUDY OF TEST PANELS
=Panel Sections for Laboratory Test.= In order to make a laboratory
study of the painted panels on the Atlantic City and Pittsburg fences,
it was thought advisable to remove small sections from representative
areas and transfer them to the laboratory for such work. The fences were
visited by the official inspection committees soon after the first
annual inspection, and the panels were carefully looked over. Upon each
was marked out a representative portion, care being exercised to select
areas where previous inspections had not disturbed the surface of the
film in any manner. The inspectors then placed the number of the panel
upon the areas which had been marked off, as well as their initials. The
marked sections were sawed out, wrapped in tissue paper, and then
transferred to the laboratory where they were placed upon models of the
respective fences from which they had been removed. The illustration
shows the model test fences set up together. It is very apparent that
the Pittsburg panels are much the darker in color, due to the soot, and
in some cases lead sulphide formed upon their surfaces. This difference
was undoubtedly due to the atmospheric conditions prevailing where the
tests were made. One would be led to suppose that a paint film exposed
to an atmosphere such as is found in Pittsburg would show deterioration
more rapidly than one exposed in Atlantic City. In all the tests and
experiments, however, the Atlantic City panels appeared broken down to a
much greater extent; though it is true that the Pittsburg panels had
darkened considerably and presented a rather mottled appearance. The
deposit of soot on the Pittsburg panel seemed to act as a preservative
coating for the film beneath, and prevented marked disintegration.
[Illustration: Sections of Atlantic City and Pittsburg Fences Arranged
for Laboratory Examination]
[Illustration: Sections of Atlantic City and Pittsburg Fences]
[Illustration: Upper set of tests made on Panels from Atlantic City
Fence
Lower set of tests made on Panels from Pittsburg Fence
Figures at left indicate Formula Number
Figures at right indicate Degree of Chalking]
[Illustration: Color Standard used in Comparison of Panel Section]
=Chalking Test.= Small strips of black felt, about one inch square, were
firmly attached to a block of wood, and by a clamp having the same
pressure in each case, the wood with its surface of black felt was fixed
to the panel. This apparatus, which resembles a blackboard eraser, is
firmly drawn across the panel in one direction for a certain definite
distance, during which time it gathers all the chalked surface presented
by the painted wood. Upon detaching the apparatus from the panel it is
observed that the black cloth becomes whitened to an extent
proportionate to the chalking that has taken place on the given area.
After each one of the panels had been treated in the same manner by the
same operator, the black cloths were assembled on one large board and
photographed. A definite standard of chalking was made up, and the
operator was enabled to put down opposite the report on each panel the
degree of chalking which had taken place, No. 1 representing the least
amount and No. 10 the greatest amount of chalking.
=Degree of Whiteness Shown by Panels.= It was a very simple matter to
gauge the whiteness of the various panels, by comparing them with a
series of standard boards painted with three coats of white paint.
Florence Brand, New Jersey zinc oxide, was used as the standard for
whiteness and termed "No. 1." In making "No. 2" standard, to the zinc
oxide was added .01% of lampblack. By adding .02% of lampblack to the
zinc, standard "No. 3" was obtained, and so on, increasing the amount of
lampblack in each case by .01%. These standards were run up to "No. 30,"
and the various panels on the different fences compared with them. The
degrees of whiteness are recorded in progressive numbers, No. 1 being
the standard for whiteness and No. 30 the darkest. The Atlantic City
panels ranged from 3 to 8 in the scale of whiteness, while the Pittsburg
panels required the use of the entire range of standards.
=Resistance to Abrasion.= The apparatus used for determining the
abrasion resistance of a paint was made of a glass tube about six feet
long, having an internal bore of 7/8 inch. This was supported in an
upright position over a dish which held the panel under test at an angle
of 45 degrees. The abrasive material consisted of No. 00 emery, which
was dropped into the tube through a funnel having a bore of 5 mm. When
the emery reached the bottom of the long tube it scattered itself so as
to strike a surface on the panel about an inch in diameter. The emery
was constantly poured in until the paint coating had worn away, showing
the bare wood. The weight in pounds of emery powder required to show the
disruption of the coating is recorded and reported as the measure of the
"abrasion resist." The panel requiring the greatest weight of emery to
cause abrasion is evidently the most resistant to abrasion. Paint is
often subjected to serious abrasion, through the blowing of sand,
especially at the seashore, and to withstand such action should contain
a proportion of pigments especially resistant to abrasion, such as
silica, zinc oxide, asbestine, and barytes.
[Illustration: Apparatus for Determining the Abrasion Resistance of
Paints]
[Illustration: Formula No. 1, A. C.]
[Illustration: Formula No. 2, A. C.]
[Illustration: Formula No. 3, A. C.]
[Illustration: Formula No. 4, A. C.]
[Illustration: Formula No. 5, A. C.]
[Illustration: Formula No. 6, A. C.]
NOTE: The author wishes to acknowledge the assistance of Dr. J.
A. Schaeffer in the preparation of the photomicrographs herewith
shown.
[Illustration: Formula No. 7, A. C.]
[Illustration: Formula No. 8, A. C.]
[Illustration: Formula No. 9, A. C.]
[Illustration: Formula No. 10, A. C.]
[Illustration: Formula No. 11, A. C.]
[Illustration: Formula No. 12, A. C.]
[Illustration: Formula No. 13, A. C.]
[Illustration: Formula No. 14, A. C.]
[Illustration: Formula No. 15, A. C.]
[Illustration: Formula No. 16, A. C.]
[Illustration: Formula No. 17, A. C.]
[Illustration: Formula No. 18, A. C.]
[Illustration: Formula No. 19, A. C.]
[Illustration: Formula No. 20, A. C.]
[Illustration: Formula No. 33, A. C.]
[Illustration: Formula No. 34, A. C.]
[Illustration: Formula No. 35, A. C.]
[Illustration: Formula No. 36, A. C.]
[Illustration: Formula No. 37, A. C.]
[Illustration: Formula No. 38, A. C.]
[Illustration: Formula No. 39, A. C.]
[Illustration: Formula No. 40, A. C.]
[Illustration: Formula No. 45, A. C.]
[Illustration: Formula No. 46, A. C.]
[Illustration: Formula No. 47, A. C.]
=Making Photomicrographs.= The photomicrographs which are herewith shown
were made in the following manner: A part of a panel was placed upon the
stage of the microscope and held firmly in place with clips. By varying
the adjustment and carefully running over the field the condition of the
surface was readily given, using the same eye-piece and objective
throughout the tests, and obtaining a magnification of thirty-three.
Great care was exercised to secure an average field showing the general
and typical appearance of every panel. Little difficulty was experienced
in so doing, as the laboratory panels gave very representative surfaces
of the large panels on the fence. The instrument was then inclined
horizontally and the eye-piece was fitted into the camera nose. In the
back of the bellows of the camera was placed the ground glass for
focusing. To secure illumination the light from an electric arc lamp
was reflected from a mirror directly upon the painted surface of the
panel, which in turn was reflected through the camera on to the ground
glass. The plate-holder was then put in position and six-second
exposures were made, afterward developing and printing.
=Checking and Cracking.= What was termed "fine matt checking" at the
First Annual Inspection was not visible at the time to certain members
of the Inspection Committee, but it is an established fact that the
checking was an existing condition, as the photomicrographs have shown.
This checking has a very peculiar characteristic in that the lines are
very narrow and hair-like, being somewhat interlaced and peculiarly
forked. That this hair matt checking is a preliminary condition which
afterwards develops into matt checking and into marked or heavy checking
seems to be indicated.
It appears from an examination of the photomicrographs of the paint
films that a paint coating closely resembles the surface of the earth,
and is subject to the same basic laws that have caused the various
geodetic changes in the earth's crust. Observation of a dried pond or
lake bed will disclose types of fissuring and cracking similar to those
shown by dried paint coatings in which the oil has been fully oxidized,
and especially in the case of paints containing pigments which act upon
the oil to produce compounds brittle in nature.
At Atlantic City the panels were all clean and free from dirt,
presenting continuous exposure of the films, and thus maintaining
conditions for active checking. At Pittsburg, soon after the panels
began to chalk, the large amount of dust and black soot in the
atmosphere completely covered the panels with a very thick, resistant
coating of carbon, which acted as a seal or protector, preventing
disintegration to a great extent. This coating was extremely hard to
remove, and photomicrographs, before and after removal of this coating
by rubbing with a damp cloth, failed to reveal marked checking on any of
the formulas except those made of strictly pure basic carbonate-white
lead. The checking, even on these, was not as marked as at Atlantic
City. It is presumed that after the chalking had taken place and the
chalked pigment had been washed from the panels, the gradually
increasing coat of carbon and lead sulphide had protected the panels
from checking, or possibly the atmosphere of Pittsburg, which in other
respects had deteriorated the panels to a greater extent than at
Atlantic City, did not have the extreme action in causing checking that
the Atlantic City atmosphere seemed to have effected.
[Illustration: Combination Formula No. 1, Pittsburg
BEFORE WASHING
Mottled surface due to external coating of impurities.]
[Illustration: AFTER WASHING]
[Illustration: Formula No. 4, Pittsburg
BEFORE WASHING]
[Illustration: AFTER WASHING]
[Illustration: Formula No. 38, Pittsburg
Basic Carbonate--White Lead Panels on Fence
BEFORE WASHING
Checking evident even through the outer covering of foreign matter.]
[Illustration: AFTER WASHING]
[Illustration: Formula No. 36, Pittsburg
Basic Carbonate--White Lead Panels on Fence
BEFORE WASHING
Peculiar network-like checking appearing through outer coat of
impurities.]
[Illustration: AFTER WASHING]
[Illustration: Formula No. 40, Pittsburg]
[Illustration: Formula No. 45, Pittsburg]
=Results on Combination Pigment Paints.= It will be noticed that the
checking on most of the combination pigment paints made of lead, zinc,
and inert pigments, was moderate, and in many cases of a fine order. It
has been observed that the percentage of zinc oxide in a paint is not
always a criterion upon which future checking may be judged. Nor could
it be said that the checking is dependent upon the percentage of basic
carbonate-white lead added to the paint. However, it appears that
scientific blending of the various pigments, with regard to their
physical properties in oil, such as their strength and elastic limit,
develops the greatest resistance to both cracking and checking.
Elasticity is vital, but strength must be combined therewith in order to
prevent disruptions of the paint coating. Paint films made of certain
inert pigments, when tested on the filmometer, were relatively high in
strength, but relatively low in elasticity. Such pigments, when used in
large percentage, form coatings which are hard and apt to crack. The
use, however, of these pigments in moderate percentages seems very
beneficial in overcoming the effect of using an excessive percentage of
white lead, or of zinc oxide.
=Results on White Lead Paints.= The maximum checking was observed on the
basic carbonate-white lead panels, the size of the checks in some cases
being several times larger than those on the other panels.
On some of the basic carbonate-white leads the checking was of a very
peculiar nature, consisting of very broad fissures in the paint coating,
disclosing the wood surfaces beneath. The type of checking existing was
also distinct in its structure, being hexagonal in shape. One of the
most marked features shown by the basic carbonate-white lead films was
the extreme roughness of their surfaces. This roughness is most likely
due to the excessive chalking which had taken place.
=Results on Silica and Barytes Paints.= The checking of paints very high
in silica resolved itself into fine hair-like lines which are generally
lateral to each other, and indicate a cracked appearance. The checking
of paints containing very high percentages of barytes was also of a
distinct nature, being generally forked in appearance and of no definite
striation.
=Surface Condition of Fume Pigment Paints.= The panels painted with
basic sulphate-white lead (sublimed white lead) showed complete absence
of checking. This was also true of the panels painted with zinc lead.
These are both fume products and are extremely fine in their physical
size, which may account for this condition. Although zinc oxide is made
in a similar manner, it gives a much harder paint coating than either of
the afore-mentioned pigments, and presents a surface which develops
considerable checking, generally of a medium order. The past theories
regarding zinc oxide, in which it has been maintained that zinc oxide
gives the maximum checking, are evidently incorrect, as the checking
found on the zinc oxide panels was not as marked or deep as the checking
on the basic carbonate-white lead panels; in fact, the checking might be
more in the line of a cracking, possibly due to the brittle nature of
the coating composed of straight zinc. This is especially true of zinc
paints containing insufficient oil.
=The Importance of the Physical Nature of Pigments.= It appears that
very fine grinding of materials, chosen for their characteristic
fineness, with the absence of any unfavorable physical condition or
chemical sensitiveness, are important factors in the making of a paint
to resist cracking or checking. The purity of the essential materials,
as well as the scientific compounding of these materials, with due
regard to the law of minimum voids, are great factors which enhance the
qualities of paints, greater, perhaps, than the variation of percentages
of the various pigments which go to make up a paint.