PHYSICAL LABORATORY PAINT TESTS
For the paint chemist who desires to familiarize himself with the more
recent analytical methods worked out in American laboratories, reference
may be had to treatises on the analysis of paints, by Gardner and
Schaeffer,[16] and Holley and Ladd.[17] Analytical methods are not
included in this chapter, the writer's desire being to treat the subject
from the standpoint of the physical properties of painting materials.
The work outlined herein is of a nature that affords a wide field of
research, and a brief study will doubtless suggest similar work to the
student of paint.
[16] The Analysis of Paints and Painting Materials. McGraw-Hill Book
Co., New York, 1910.
[17] Mixed Paints, Color Pigments and Varnishes. John Wiley & Sons,
New York, 1908.
=Preparation of Paint Films.= The study of paint films is one that has
become of vital importance, and is receiving at the present time great
attention. Among the many methods which have been suggested and
attempted for securing paint films, a few already well known may be
mentioned.
By painting upon zinc and eating away the zinc with acid: The objection
to this method is very evident, namely, the action of the acid upon the
paint coating, which is likely to be very severe. Another method has
been to spread paraffin on a glass plate, and painting upon this
surface. When the paint is dried, the paraffin is melted off and thus
the film is obtained. This method is open to objections, in that the
paraffin surface is not a comparable one upon which to paint, and also
that the complete removal of the paraffin is not assured.
Another method consists in covering a piece of glass with tin foil,
painting out the film upon the foil, and after drying properly, to
remove the sheet of foil with its coating of paint and immerse in a bath
of mercury which, by amalgamation of the tin, leaves the paint film.
We now come to a method worked out in our laboratories, which can be
recommended as being not only simple but efficient and practical. It has
been found that a size from noodle glue, when painted upon ordinary
fair-quality paper, makes a surface from which the paint may be
subsequently stripped. The paint is applied in the ordinary way to the
paper, which is held during the operation by thumb tacks, and allowed to
dry. The paint may be separated by immersion in water kept at about 50
degrees Centigrade. By this method large films may be obtained, but it
has been found very unhandy to work with films exceeding an area of
eight inches square. When the film of paint has been detached from the
sized paper through the dissolving of the noodle glue, the paint film is
then immersed in a fresh solution of water, in order to remove whatever
excess of noodle glue there may be remaining. A glass rod is then
introduced into the bath, in which the paint film is floated upon the
glass rod, which is then hung up to dry in a suitable container to
prevent the accumulation of dust, etc.
[Illustration: Bottles Showing Relative Permeability of Films by Amount
of Whiting Formed Within]
=The Permeability of Paint Films.= A series of tests were made to
determine the water-excluding values of various combinations of painting
pigments ground in pure linseed oil. White pine boards, six inches long,
four inches wide, and one inch thick, were carefully prepared and
numbered and given three coats of a white paint formula of the
corresponding number. After drying, the boards were carefully weighed
and immersed in a tub of water for three weeks. After removal, the
surfaces of the boards were dried with blotting paper and the boards
weighed. The gain in weight, corresponding to the amount of water
penetrating through the pores of the wood, was observed. The boards were
again immersed and at the end of two months the following results were
obtained:
Grammes of water
Formula absorbed
No. through paint
1. Soya bean oil 120
2. Linseed oil 102
3. Calcium sulphate 93
4. Barytes 88
5. Asbestine 74
6. Corroded white lead 59
{ Basic carb.--White lead 25% }
{ Basic sulph.--White lead 20% }
7. { Zinc oxide 25% } 58
{ Calcium sulphate 25% }
{ Calcium carbonate 5% }
8. Sublimed white lead 56
9. Zinc oxide 56
{ Zinc lead white 30% }
10. { Zinc oxide 40% } 42
{ Basic carb.--White lead 20% }
{ Calcium carbonate 10% }
11. { Basic carb.--White lead 50% } 42
{ Zinc oxide 50% }
{ Basic carb.--White lead 38% }
12. { Zinc oxide 48% } 38
{ Silica 14% }
The test boards were then exposed, with their content of water, to the
action of the sun's rays. Blistering of the painted surfaces took place
in many cases, caused by the rapid withdrawal of the water and its
consequent action on the paint film. The tests seem to indicate that a
mixture of white lead and zinc oxide, with or without a small percentage
of the inert pigments, is not as subject to the action of the water as
the single pigment paints. In order, however, to corroborate these
tests, it occurred to the writer to develop a more visible means of
demonstrating the passage of moisture through paint films.
[Illustration: Bell Jar Apparatus for Testing Permeability of Paint
Films
Paint films sealed over mouths of Bottles containing Lime Water.
Carbonic Acid Gas generated under Bell Jar passes through Plate Films
and precipitates Calcium Carbonate]
Another series of white pine boards were therefore soaked in a solution
of iron sulphate for several hours. After removal, the surface of each
board was dried and coated with one coat of the paints previously
tested. After thorough drying for forty-eight hours, there was placed on
the surface of each board a few drops of a solution of potassium
ferrocyanide. This solution has the effect of producing a blue
coloration with iron sulphate, and in every case when it was placed on a
paint of considerable porosity, the solution penetrated through and
formed a blue coloration beneath the paint. The results corroborated the
original tests referred to above.
A series of sheets or films of paints were then prepared according to
the method referred to on page 71. These films were placed over glass
dialyzing cups, allowing the inner surfaces to sag so as to hold a small
amount of dilute ammonium chloride solution. Distilled water was placed
on the reverse side of the dialyzing apparatus and the tests started. At
the end of six days the distilled water in each test was examined and
the following results obtained:
Test No. 1 (corroded white lead and asbestine film) allowed the
passage of 0.002 gm. ammonium chloride. Test No. 2 (corroded
white lead and zinc oxide film) allowed the passage of 0.0003 gm.
ammonium chloride.
Tests were also made with dilute solutions of other salts such as ferric
chloride, having a dilute solution of potassium sulpho-cyanide on the
reverse side of the apparatus. In the latter case the formation of a
pink color, characteristic upon the mingling of these solutions, was
obtained in a few hours.
=Film-Testing Machine.= A film-testing apparatus, termed a "filmometer"
by its originator, Mr. R. S. Perry, was constructed, with the following
features: A graduated upright tube is fixed by means of sealing wax to
two metallic plates which carry an evenly bored hole, exactly under the
hole in the upright tube. This hole measures exactly one square
centimeter in area, and is circular. The upright tube is graduated into
lineal centimeters and is called the pressure tube.
[Illustration: Gardner Accelerated Test Box]
[Illustration: Perry Film Testing Machine]
Attached to the lower end of this pressure tube, close to the metallic
plates which serve as carriers for the paint film to be tested, is a
side-neck, which is inclined at an angle of 45 degrees to the pressure
tube, and serves the purpose of introducing the mercury, as will be
described later. Immediately under the openings in the metallic plates
which carry the film are arranged two pieces of iron inclined at a
90-degree angle, so arranged that when the pressure of mercury is
applied and causes rupture of the film, the falling mercury shall be
caught between these two insulated plates and cause contact. These two
plates are connected up by wire with a pair of magnets, thence to an
electric bell, and from there to storage batteries which supply the
current.
A film of paint is tested in the following manner: A piece of film one
inch square is cut out and placed between the two metallic plates which
hold the film immediately under the pressure tube. Mercury is run in
from a burette through the side-neck and applies pressure upon the film
by gravity. As the mercury is run in it rises of course in the tubes
until this pressure becomes so great as to finally break the film. At
this point the mercury will run out, and, falling upon the two insulated
iron plates immediately below, will cause contact and close the circuit
which rings an electric bell, which is a signal for the operator to shut
off the inflow of mercury through the side-neck from the burette.
The pressure tube is also supplied with a piston which is made of a
piece of thin iron wire having a disc attached to its lower end. As the
mercury rises in the pressure tube this iron wire is pushed up, being
very delicately counterpoised over a wheel. Upon the breaking of the
film the mercury runs out, but upon falling upon the two iron plates
underneath causes contact to be made, which causes the current to run
through the pair of magnets before mentioned, which, becoming
electrified, attract the piston in the pressure tube, giving a reading
for the maximum height of the column of mercury.
[Illustration: Diagram of Perry Filmometer]
The supply of mercury being shut off, the operator is now in a position
to determine the total sum of both the elasticity and ductility of the
paint film, and also the pressure at which the film broke. The breaking
pressure of course is read directly upon the pressure column, which is
divided into centimeters as has been described above, the piston
indicating the maximum height of the mercury column. What may be termed
the elasticity of the film can now be calculated. As is perfectly
evident, the film in stretching does so by distending from a flat
surface to a curved or cup-like surface. If the pressure tube is
calibrated in cubic centimeters reckoned from a flat surface where the
film was introduced, the stretch of the paint film in distending from a
flat surface to a curved surface may be determined. The cubic contents
of the pressure tube and side-arm become increased, owing to the
cup-like shape the paint film takes on. By subtracting the amount of
mercury indicated by the piston in the pressure tube from the amount of
mercury delivered from the burette, the amount contained in the
distended paint film is obtained, which serves as a measure of
elasticity. The temperature is a most important point to consider in
running daily tests upon the filmometer. The tests made by the writer
were conducted at 70 degrees Fahrenheit throughout.
[Illustration: Gardner-de Horvath film testing apparatus]
=Gardner-de Horvath Filmometer.= Another type of filmometer which gives
very concordant results was recently devised by the writer and de
Horvath. This apparatus is shown above.
It consists of a three-necked Wolff bottle having provision at one of
its necks for exhausting the air from the bottle. The reverse neck is
provided with a gauged glass tube dipping into a porcelain crucible
containing mercury, thus acting as a manometer. The middle neck is
fitted to accommodate two ground glass plates. Both these plates are
provided with a central orifice one millimeter in diameter. Between the
plates is placed a small section of paint film. The plates may be
pressed together or clamped together and placed over the middle neck of
the bottle, a close contact being made with Canada balsam. As the air is
exhausted from the bottle, the mercury in the tube will rise and
continue in its ascent until the film, which is exposed to atmospheric
pressure, has offered it maximum resistance, which is shown by the
breaking point. This point is observed on the manometer and the result
expressed in centimeters of mercury.
=Table of Film Testing Results.= By means of the Perry film-testing
apparatus, described in the above, interesting results have been
obtained, which are embodied in the following table:
COMPARATIVE STRENGTHS OF FILMS AS OBTAINED BY THE BREAKING MACHINE
============================+=========+==========+===========+========
|No. Coats| Pressure | Thickness | Stretch
----------------------------+---------+----------+-----------+--------
1. Zinc oxide | 3 | 33.2 | 0028 | .30
2. Zinc lead | 3 | 32.7 | 0034 | .35
3. Asbestine | 3 | 28.0 | 0045 | .15
4. Sublimed white lead | 3 | 17.9 | 0024 | .38
5. Barytes | 3 | 13.3 | 0042 | .33
6. Lithopone | 3 | 13.1 | 0024 | .49
7. Whiting | 3 | 13.0 | 0033 | .32
8. Quick process white lead| 3 | 11.3 | 0025 | .38
9. Gypsum | 3 | 10.8 | 0039 | .29
10. China clay | 3 | 10.8 | 0035 | .16
11. Silex | 3 | 9.6 | 0032 | .32
12. Blanc fixe | 3 | 8.5 | 0030 | .28
13. Corroded white lead | 3 | 7.3 | 0020 | .33
14. Barium carbonate | 3 | 7.2 | 0028 | .16
============================+=========+==========+===========+========
By means of this machine it is possible to obtain very valuable
information concerning the effect of age upon a paint as influencing its
strength and elasticity. These are two vital qualities in a paint, as it
is through its strength that a paint resists abrasion, cracking,
peeling, and blistering. That elasticity is a vital qualification of a
paint may easily be seen through the checking of oil paintings, which,
as Ostwalt has pointed out, is due to the unequal coefficients of
expansion between the ground and the paint. This is particularly
noticeable in the alligatoring of many enamels which contain large
percentages of zinc.
Curves have been prepared having pressure as an abscissa and elasticity
as ordinate. These curves show remarkable differences in different
pigments. For instance, in the case of white lead, the curve takes a
steep upward trend when it apparently reaches a maximum, the curve then
flattening out and finally taking another steep upward trend just before
breaking. This may be construed as follows: That under low pressures the
white lead film is perfectly elastic, when a maximum is obtained, beyond
which elasticity does not extend. This point is the maximum point of the
upward trend. From here on pressure may be applied without any increase
in stretch, this being represented by the flat part of the curve, while
the steep upward trend just before breaking shows where the paint begins
to tear, finally culminating in breaking. In the case of asbestine,
however, the curve is more of a straight line up to the breaking point,
which would go to prove that elasticity is proportionate to pressure in
the case of this pigment.
=Moisture Absorption.= The structure of certain pigments is such that
when they are ground in linseed oil and painted out, films are produced
which are very water-resistant. This action is possibly due to the
filling of the voids in the oil, thus making a compact and
water-resistant film. Pigments which are coarse and which present an
angular crystalline structure, often produce films which contain a
relatively large number of voids and are less waterproof. Certain
pigments are chemically active and tend to produce, when ground in oil,
metallic soaps which act for a time more or less as varnish gums, in
keeping out moisture. Later on, however, such films are apt to break
down and admit moisture in quantity. The tests herein described were
designed by the author to determine the water-excluding value of a
number of typical pigments when ground in linseed oil and painted out
into films. Unfortunately, no method has been devised by which films of
the same gauge could be prepared. The variations in the thickness of the
films used in these experiments, however, are not very great.
[Illustration: Apparatus for Determining Excluding Properties of Paint
Films]
A series of small glass bottles with wide mouths, holding about two
ounces, were half filled with concentrated sulphuric acid, and paint
films were tightly sealed over the mouths of the bottles with Canada
balsam. The bottles were then carefully labeled, numbered, and
accurately weighed upon chemical balances. Later they were exposed under
a large glass bell jar containing air saturated with moisture and kept
at a constant temperature. The bottles were removed from the receptacle
every week and reweighed. The increase in weight, due to the amount of
moisture which had penetrated through the films, and absorbed by the
sulphuric acid, owing to its hygroscopic nature, was thus determined. In
another series of bottles, lumps of calcium chloride were substituted
for the sulphuric acid. The results obtained from these tests correspond
to those of the former tests, and led to the conclusion that the
porosity of linseed oil films varied when different pigments were used
in the oil.
MOISTURE EXPERIMENTS
Figures Given Express Percentage Gain in Weight, e.g., Water Absorbed
==========================+=========+=========+=========
| 7 days | 21 days | 49 days
--------------------------+---------+---------+---------
White lead and zinc oxide | 0.043% | 0.115% | 0.266%
Zinc lead white | 0.049 | 0.130 | 0.284
Red lead | 0.049 | 0.130 | 0.295
Sublimed white lead | 0.049 | 0.128 | 0.292
Zinc chromate | 0.064 | 0.176 | 0.417
Zinc oxide | 0.065 | 0.172 | 0.391
Barytes | 0.074 | 0.202 | 0.466
Willow charcoal | 0.077 | 0.236 | 0.694
Lithopone | 0.083 | 0.228 | 0.550
Chinese blue | 0.092 | 0.276 | 0.671
Natural graphite | 0.104 | 0.350 | 0.951
Ultramarine | 0.119 | 0.336 | 0.814
==========================+=========+=========+=========
Another series of tests was started, in which were used films prepared
from various oils and varnishes made especially for the test from
different gums. The results of this series are very interesting, as they
indicate that certain gums are more powerful than others in making oils
resistant to moisture. The reader should study with care the data on
treated Chinese wood oil, most excellent results having been obtained
when it was used in the proper percentage.
EXCLUDING TESTS ON OIL VEHICLES AND VARNISHES SHOWING PERCENTAGE OF
MOISTURE ABSORBED AT VARIOUS PERIODS
===================================+=========+=========+=========
| 6 days | 18 days | 24 days
-----------------------------------+---------+---------+---------
Linseed oil, 100% | .233 | .686 | .895
Soya bean oil, 100% | .340 | 1.06 | 1.39
Linseed oil, 80% } | .250 | .755 | .987
Soya bean oil, 20%} | | |
Linseed oil, 60% } | .289 | .857 | 1.125
Soya bean oil, 40% } | | |
Linseed oil, 40% } | .355 | 1.05 | 1.39
Soya bean oil, 60%} | | |
Linseed oil, 20% } | .260 | .789 | 1.03
Soya bean oil, 80% } | | |
China wood oil treated, 100% | .130 | .297 | .375
Linseed oil, 80% } | .182 | .559 | .728
China wood oil treated, 20%} | | |
Linseed oil, 60% } | .173 | .540 | .708
China wood oil treated, 40% } | | |
Linseed oil, 40% } | .119 | .348 | .450
China wood oil treated, 60%} | | |
Linseed oil, 20% } | .127 | .375 | .494
China wood oil treated, 80% } | | |
Kauri gum, 33% } | | |
Linseed oil, 33%} | .061 | .191 | .302
Turpentine, 33% } | | |
Kauri gum, 25% } | | |
Linseed oil, 50% } | .096 | .266 | .346
Turpentine, 25% } | | |
Kauri gum, 20% } | | |
Linseed oil, 60%} | .122 | .367 | .449
Turpentine, 20% } | | |
Kauri gum, 15% } | | |
Linseed oil, 70% } | .187 | .421 | .601
Turpentine, 15% } | | |
Congo copal gum, 20% } | | |
Linseed oil, 50% } | .228 | -- | --
Turpentine, 30% } | | |
Sierra Leone copal, 20% } | | |
Linseed oil, 50% } | .099 | -- | --
Turpentine, 30% } | | |
Zanzibar gum, 20% } | | |
Linseed oil, 50% } | .082 | -- | --
Turpentine, 30% } | | |
Amimi gum, 20% } | | |
Linseed oil, 50% } | .080 | -- | --
Turpentine, 30% } | | |
Boiled linseed oil (linoleate type)| .210 | -- | --
Collodion solution (6 oz.), 80% } | .201 | -- | --
Boiled linseed oil, 20% } | | |
===================================+=========+=========+=========
[Illustration: Microscopic view of section of cedar]
[Illustration: Microscopic view of section of maple]
[Illustration: Microscopic view of section of white pine]
[Illustration: Gardner photomicroscope in position against painted
surface]
[Illustration: Inside White on White Pine]
=Use of the Microscope.= 4. The microscope is a necessary adjunct of
every well-ordered paint laboratory, as has been recognized throughout
the whole paint industry. The writer has attempted to collect certain
data which may materially assist those manufacturers who employ this
instrument to judge of the quality of their raw and finished products.
The fineness of grinding considerably affects the quality of the paint,
and this can be easily controlled through the intelligent use of the
microscope. This instrument may also be used to detect certain
adulterations. Appended is a table giving the fineness of grinding of
the various pigments, together with their characteristics under the
microscope. In this table measurements are given both in millimeters and
in inches, in order to accommodate itself to the use of those chemists
employing a millimeter stage micrometer, or those employing the English
or inch system. Although it is not yet certain that any and all
combinations of pigments may be detected under the microscope the writer
is working toward a method which will allow a manipulator to judge of
the composition of the paint under observation.
In order to properly prepare a paint for microscopic examination, the
following method is recommended: A microscopic turn-table is a
convenient accessory of the microscope, and its use is to be
recommended. A glass slide being placed in position upon the turn-table,
a very small amount of either the pigment rubbed up in oil, or the
paint, is applied to the slide; a small drop of Canada balsam is then
applied by means of a glass rod dipped in a solution of balsam in xylol,
and dropped upon the slide. The rod is then used to thoroughly
incorporate the pigment with the balsam, and a cleaned cover glass is
dropped over the whole and pressed down tightly, so that a small amount
of balsam will exude from under the edges and thus firmly seal the
glass. In order to make permanent slides it has been found advisable to
rim the cover glass with balsam and even follow this up with some
suitable black varnish, the slide being then carefully labeled and
placed in the collection. Following is a table of the characteristics of
the fourteen chief pigments:
TABLE OF THE SIZE OF PARTICLES OF THE CHIEF PIGMENTS WITH THEIR
CHARACTERISTICS UNDER THE MICROSCOPE
===+===================+======================+=======================
| | Diameter in | Diameter in
| | Millimeters | Inches
| +-------+-------+------+-------+------+--------
No.|Name | Small | Aver. |Large | Small | Aver.| Large
---+-------------------+-------+-------+------+-------+------+--------
1|Asbestine |.002 | -- |.12 |.00015 | -- |.049
2|China clay |.003 | -- |.065 |.00009 | -- |.025
3|Barium carbonate |.00076 |.0055 |.0172 |.00003 |.00024|.0011
4|Blanc fixe |.00073 |.0037 |.0073 |.00003 |.00014|.0003
5|Silex |.0037 |.0092 |.03 |.00014 |.00036|.0012
6|Gypsum |.0037 |.011 |.05 |.00014 |.00044|.0022
7|Amer.-Paris white |.0015 |.0050 |.04 |.00006 |.00022|.0018
8|Barytes |.0015 |.0092 |.05 |.00006 |.00036|.0021
9|Zinc lead |.00037 |.0018 |.0037 |.000014|.00007|.00014
10|Sublimed white lead|.00037 |.0018 |.0037 |.000014|.00007|.00014
11|Lithopone |.00076 |.0018 | -- |.00003 |.00007| --
12|Zinc oxide |.00046 |.0018 |.00037|.00002 |.00007|.00014
13|Quick Pro. lead |.00061 |.0030 |.0048 |.00002 |.00012|.00018
14|Dutch Pro. lead |.00061 |.0018 |.0066 |.00002 |.00007|.00026
===+===================+=======+=======+======+=======+======+========
=Film Sectioning and Deductions to be Drawn Therefrom.= 5.
Investigations were undertaken into the innermost structure of paint
films as revealed under the microscope. Up to the present time, work has
been done upon barytes, asbestine, blanc fixe, and white lead, painted
upon wood, and a combination paint upon wood. The films, the preparation
of which has been described in the foregoing, were sectioned and
prepared for microscopic examination in the following manner:
A solidifying dish was partly filled with low melting-point paraffin
which was allowed to harden, when a small piece of paint was thrown upon
it and then more paraffin poured over it. After hardening, sections were
obtained of the paint film by means of a microtome.
[Illustration: Section Barytes Film]
A view of these sections of paint films under the microscope gave to the
operator a better idea of the structure of a paint than had ever been
afforded heretofore. It was easy to perceive the relative position of
the pigment particles and the three coats. The penetration of one coat
into another was easily discernible, and measurements were made upon the
sections in order to determine the average thickness of coat and its
general appearance.
Sections were also made of Inside and Outside White upon wood. These
sections revealed under the microscope the thickness of the coats and
also the penetration of the priming coat into the wood. Appended is a
table giving microscopic measurements.
PAINT SECTION MEASUREMENTS UNDER MICROSCOPE
======================+=============+===========+======
| |Millimeters|Inches
----------------------+-------------+-----------+------
Barytes |3 coats (sum)| .1068 |.00421
|Single coat | .0356 |.00140
| | |
Inside. White on wood |3 coats (sum)| .1624 |.00639
|Outside coat | .0230 |.00091
|Next coat | .0443 |.00175
Field in photographs |Next coat | .0620 |.00245
|Penetration | .0294 |.00116
White lead |Inside | .0215 |.00085
|Middle | .0405 |.00159
|Outside | .0184 |.00073
|3 coats (sum)| .0811 |.00319
Asbestine |3 coats (sum)| .1840 |.00725
| | |
Blanc fixe |3 coats (sum)| .1068 |.0042
|Single coat | .0356 |.00014
| | |
Outside. White on wood|Outside coat | .1329 |.00523
|Inside | .1845 |.00727
|Penetration | .0737 |.00290
======================+=============+===========+======
=Polar Micro-Examinations and Photomicrographs.= By Polar
Micro-Examination is meant the examination of pigments under polarized
light. A polarizing apparatus, though not an essential in the hands of
the paint chemist, is nevertheless much to be desired, for by its help
deductions may be drawn as to the contents of a paint, which by other
means might not be possible. The polarizing apparatus as marketed by
most manufacturers of the microscope is attached in the following
manner:
The diaphragm immediately under the sub-stage container is swung out and
opened to its widest limit, allowing the insertion of the polarizer.
This polarizer carries one of the pair of Nicols prisms and is
countersunk to allow of the introduction of gypsum or selenite plates.
The analyzer fits over the eyepiece on the tube.
The use of polarized light upon paint is valuable on account of its
action upon crystalline substances. The re-enforcing pigments, such as
Asbestine, China Clay, Gypsum, Silex, Barytes, etc., are crystalline and
consequently act upon the polarized light. In most cases these pigments
are used in ready-mixed paints in small amounts, varying between 5 and
25%. When a slide containing a small amount--for example, less than
3%--of these crystalline pigments is examined under the microscope by
ordinary transmitted light, they will often escape observation, owing to
the small amount in which they are present. However, in the case of
polarized light, this could hardly happen.
[Illustration: Microscopic View of Barytes under Polarized Light]
A slide of paint containing these re-enforcing pigments is prepared in
the usual manner. On examining this under the microscope and using the
polarizing apparatus, the crystalline pigments are at once detected by
revolving the analyzer. At one position of the analyzer, one sees an
ordinary field, as with transmitted light, but if one revolves the
analyzer, the field gradually becomes darker until total darkness is
obtained throughout, except in such places where crystalline substances
are present, when the crystal is shown up with beautiful distinctness.
Photomicrographs of various single pigments and pigment combinations are
shown under Chapter III.
=Effect of Pigments on Oil.= Certain pigments have the property of
acting upon the linseed oil in which they are ground, forming metallic
linoleates which accelerate the drying of oil. This is especially true
of lead and zinc pigments. The inert crystalline pigments, when ground
in linseed oil and painted out, distribute the oil so as to allow a
great surface to be exposed to the air. Thus by physical action, and
possibly catalytic or contact action, these inert pigments stimulate the
drying of oil paints in which they are ground. Lead and zinc paints, of
course, have the greatest drying values on account of the added effect
of the linoleates formed, as outlined above. The writer has made a
series of tests in which the action of various pigments upon linseed oil
is shown. The tests were made in the following manner:
Five grams of each of a series of commonly used paint pigments,
including those of inert crystalline nature as well as the more valuable
amorphous pigments which are considered more or less chemically active,
were ground separately in an agate mortar, with 5 grams of raw linseed
oil. The ground paste in each case was placed in a marked glass beaker,
and allowed to stand in a dustless section of the laboratory for one
month. The oil-pigment paste from each beaker was then separately
extracted with benzine to remove the linseed oil from the pigment. The
benzine solutions of oil were then heated to remove the benzine and the
residue of oil burned to ash in crucibles. The ash from each test was
weighed, and if it ran above the percentage of ash determined on a blank
sample of linseed oil (namely, .003%), the ash was analyzed
qualitatively for metallic constituents. The following table of results
shows the percentage increase in ash, as well as the constituents of ash
on the various samples tested:
TABLE OF RESULTS
===============================+==============+========================
| Per cent. of |
| Ash in Oil |
Pigment in Oil |Extracted from|Analysis of Ash
| Oil-Pigment |
| Paste |
-------------------------------+--------------+------------------------
Raw linseed oil without pigment| 0.003 | --
Barytes | 0.003 | --
Blanc fixe | 0.003 | --
Silica | 0.003 | --
Asbestine | 0.005 | --
China clay | 0.007 | --
Whiting | 0.008 | --
Chrome yellow | 0.025 |Lead oxide (PbO)
Lithopone | 0.031 |Zinc oxide (ZnO)
Prussian blue | 0.032 |Iron oxide (Fe_{2}O_{3})
Sublimed white lead | 0.033 |Lead oxide (PbO)
Zinc oxide | 0.105 |Zinc oxide (ZnO)
Corroded white lead | 0.116 |Lead oxide (PbO)
Red lead | 0.2112 |Lead oxide (PbO)
===============================+==============+========================
Observation of these results shows that pigments such as Barytes, Blanc
Fixe, and Silica have no chemical action on the linseed oil. The results
on Asbestine and China Clay also are negative, the extremely slight
increase in amount of ash from these samples probably being due to
traces carried over mechanically into the oil mixture; the last named
pigments being more fluffy and difficult to separate from oil. Slight
action seemed to be apparent in the case of whiting, a pigment somewhat
alkaline in nature. A longer test might have shown this pigment to have
possessed still greater action. Corroded white lead showed considerable
action, resulting in the formation of lead linoleate or some other
organic compound. Zinc oxide and lithopone, the latter pigment
containing 30% of zinc sulphide, both indicated action on the oil.
Chrome yellow (chromate of lead) showed some action, as did also
Prussian blue, the ash from the last named pigment showing a heavy
percentage of iron oxide.
Red Lead showed a most astounding gain in these tests, chemical action
of the pigment on the oil being apparent soon after the tests were
started, as shown by the formation of a hard cake with the linseed oil.
The Raw Linseed Oil which was used in these tests had an acid value of
1.84%, which is very low. The neutralization of this free fatty acid by
some of the alkaline pigments used, may account for part of the
increased percentage of ash, but in most cases the pigments, and more
especially the basic pigments, had a direct saponifying action upon the
glycerides of the oil.