|
|
Testing paints How to check how good things are
So you have just made some beautiful new paint. You are certain it is
the best paint ever made and you want to show the world how good it is
beside the paint you used to buy. How do you prove it? Or perhaps your
goal is humbler, you need to be certain that you have either enough, or
not too much binder in the paint. Perhaps you have a new colored clay
you just dug from a river bank and the color is so brilliant you
suspect it is not all iron oxide? Or maybe the manufacturer is using
marketing speak when describing a new super yellow, and you need to
find out if the claims match the reality. Then in each of these cases
you need to test. This page shows you how.
The testing procedures here are simple tests relatively easy for most
artists to conduct and offer a convenient rule of thumb. Testing using
calibrated scientific instruments is an expensive process best left to
the experts. The Yellow Pages can help you find the relevant testing
laboratories in your area should this prove necessary, Golden paints
offers this service to artist's as well. but for most
practical purposes the following are adequate for studio based paint
making. After all, if you were to sell paint more rigorous testing
would be in order but for your own use you just need to be assured that
the paint you are using will not fade in a hundred years (or within
months as is the case with the worst pigments) and that the paint film
is not going to disintegrate disastrously down the track. Occasionally
you may want to test a manufactured paint because you think that they
are adding too many fillers. A simple drawdown will reveal a surprising
amount about paints and is the easiest of tests to do.
Record keeping Your memory
is not perfect
You need a note book that is sturdy enough to last years of use that
you always keep with your pigment supply. The first half should include
your notes of how and when you mixed individual colors. There is often
a
time lag between the use of a color and the discovery that this one
needs to be repeated and the actual making. It is very difficult to
remember every detail. Keep notes that become your recipes for the
future. The second half should be your testing notes. Some tests such
as light fastness are conducted over months and years. It is impossible
to remember dates with accuracy after a certain time. For meaning full
results good record keeping is essential. Transferring the results to
computer software can be useful, but your studio notebook remains the
back up for everything.
Formulation Are the
ingredients correct?
Extremes of poor formulation are obvious during grinding, but as you
mull it may be necessary to take a closer look by
scraping the oil paint mass together with the palette knife and noting
it's ability to retain its shape without sagging or running in the case
of oil paint. A small
portion can also be placed on a piece of paper towel. Excess oil may
become visible as it is absorbed by the paper and grows as a ring
around
the dab of paint. It is possible at this point to add more pigment
directly to the paint being ground via a clean palette knife and gently
sprinkling the pigment particles over the paint mass. Grinding time
needs to be extended as if the grinding process had just begun to
ensure that all the new pigment particles are evenly coated in oil. Too
much pigment will become quickly apparent in the form of a crumbly
paint mass. Oil should be added a drop or two at a time until the paint
has the desired combination of buttery body and brushability.
Defects can also be revealed by brushing out some of the new paint
thinly on a gesso surface. Flocculation and agglomeration where the
pigment particles clump together as small lumps should be revealed by
this test. Streakiness should also be apparent at this time if it is
present.
Other paint media will have their own characteristics. Egg Tempera and
Gouache for instance should have the consistency of cream, Both
Watercolor and Acrylic will have a wider range of possibilities
depending on the desired end result. All can be examined by brushing
out and examining the resultant film.
Egg Tempera should be dabbed on a piece of glass and let dry to see if
enough egg yolk is in the mixture. Try the same test for acrylic as
well if you think you may be pushing the pigment concentration too much.
Light fastness Does your
color change with time?
Light fastness tests are fairly easy to do and can give some surprising
results. Some colors fade, others can darken and the same pigment can
behave differently in one medium compared to another, so results from
one should not be seen as evidence for another. One of these
differences stems from the protection that oil films (and acrylic) can
give to pigments, meaning that a pigment that is permanent in oil and
acrylic can be fugitive in watercolor. Some pigments, especially azo
yellows are more light fast in acrylic than in oil. Testing individual
pigments in individual media is the only way to resolve this question
for individual colors. When looking at light fastness information
provided by manufacturers take note of the paint vehicle the testing
information is based on if available.
Cardboard well prepared with acrylic gesso front back and sides is
suitable for most testing purposes. Outdoor testing is best because
results are obtained quickly and then can be calculated to indoor light
equivalents. The dashboard of a car is surprisingly
good for the purpose if it is an older car that has a convenient shelf,
but it is better to
place the card under glass in a purpose made tester as described below.
Always record dates and pigment names and tint proportions beside
test paint samples as it is easy to forget. Further information
should be recorded in your color book in the studio.
The testing can be carried out in the rough and ready way described
below with Alizarin Crimson used as the 'standard' for fugitive. It
works but is only a rough guide with many variables. Far preferable is
to use Blue Woolscale cards which calibrate exact rates of fading
accurately and can be used to assign the equivalent of the ASTM classes
to your colors. These are no more difficult to use (in fact in many
ways make things easier) than the rougher method and are recommended
for those who are very serious about testing their materials. The
Woolscale and equivalent fade resistance (in years and compared to ASTM
classes) is given below.
(1) Basic method
Make the test cards
Paint the color thinly and smoothly on the card in a strip about 40 mm
wide. make 2 more painted strips as tints, the first with 25% white
added, the second with 25% color added to white. This is because we
most commonly use colors as mixtures with other colors and often with
white. Yet most pigments are far less light fast as tints than as full
strength color. These tests as tints give an excellent idea of how well
the color stands up in the concentrations you are likely to use when
actually painting. it is useful to have a comparison strip of a known
ASTM l l l color such as Alizarin Crimson in tint on the card as well.
Several colors may be tested on the one card. Partially
cover the strips of color with a non rusting metal or other material
that is totally impervious to the light and is not too thick (to cause
shadow at the edge). It should have a sharp edge so that there is a
sharp division between exposed and non exposed. so that some of the
test strip gets light and the other part does not. After an extended
period visual inspection will reveal if there is a discernible
difference between the exposed and non exposed parts. Great care must
be exercised in replacing the strip in the identical position if
the test card is to be left for further exposure.
Test duration and
interpretation
Useful results in full sunlight can be expected within weeks for
fugitive colors, six months for more durable ones. A year is excellent.
Full sun
is far stronger than interior light and 1 year in the sun is likely to
be the equivalent of as much as a hundred years of indoor exposure.
There is some variability in this depending on your distance from the
equator and season. Any test results you may choose to leave in normal
light in the studio may be left for
a number of years before checking and noting any change if any. In
normal light on the studio wall there should be no change in the colors
at any point of a usual test period. It is reasonable to expect
no change after 50 years if you could wait that long. Any color
that noticeably changes in 50 years or less of normal interior light
should be regarded as fugitive. Be aware that there are many colors in
existence that noticeably change in 5 to 10 years of indoor exposure
and some that change in a matter of months indoors.
For accurate tints use a set of kitchen measuring spoons, the sort that
offer 1/4 spoon, 1/2 spoon and so on. In the size that will make
sufficient paint for your tests use a palette knife to fill the
measuring spoon and level it with the blade of the knife. Use 1 spoon
of white and 3 of color to get the 25% reduction, and 3 of white, 1 of
color for the 75% reduction.
The tests need to be carried out outdoors and under glass to protect
from moisture. In the tropics it may be kept horizontal but the further
from the equator you are located the more the test board should be art
an angle facing south in the northern hemisphere and facing north in
the southern hemisphere. It can be as simple as a board leaning against
a shed with a glass sheet covering it and light plywood sides siliconed
to the glass to stop rain getting to the test cards. Where the test is
of very water sensitive media such as watercolors then extra care needs
to be given to weather protection and the possibility of condensation
under the glass. Fix it so that it is sturdy enough not to blow down in
any wind, and that children and animals cannot easily get in to move or
damage the cards.
Making do
For those without the luxury of a garden space to set up a test and
(like me) have a garret on the top floor of an inner city building you
need to make do and adjust interpretations of results accordingly. If
you can set up your test cards under glass at a studio window that
receives full sunlight, preferably a south facing window in the
Northern Hemisphere, and a North facing window in the Southern. Time
how much of the day the cards get full sunlight and work out the
percentage of the average daylight hours. Maybe it is 50%, or only 30%
or some other portion. You then have an amount by which to increase the
time of exposure and interpret accordingly. The same rule of thumb
applies if your only sunlit window is east or west facing. If you have
no window that gets full sun then you may need to transfer your tests
to your parents or girlfriend/boyfriend's place as can be arranged.
(2) Using Blue
Woolscale
As has been pointed out there are many variables in total exposure
depending on where you live and how much direct sunlight you can get.
The season can also make a big difference. Blue Woolscale overcomes
this by allowing you to judge the total light exposure rather than
using a simple rule of thumb based on guesses about light intensity.
The test cards have 8 strips of wool on them each died with a different
dye that fades at a known rate from #1 the most fugitive to #8 the most
light fast. By exposing a Blue Woolscale card alongside your paint
samples with one half of the Woolscale covered just as with the paint
samples it is easy to judge actual light fastness no matter where you
are located as the Woolscale records the total light accumulated and is
not time dependent. It is still better to use full sun as the test will
finish sooner, but in fact this test will work in a wide variety of
light situations. Prepare the samples and place them under glass as
described above. The only difference is your scientifically calibrated
fading card. Cards can be purchased for around $6 to $10 each depending
on quantity bought. Golden Paints sells a testing kit that utilizes
Blue Woolscale for around $30 for an excellent kit that includes full
instructions and materials that make testing easy.
Light fastness Ratings and what they mean
Now to the contentious theme of
putting an actual lifetime value on light fastness ratings. There is no
easy answer here and most authorities try to dodge the issue. The
reason being that pictures are shown in such a wide range of
conditions. Even within one room one wall may get double (or more) the
light of another wall, effectively doubling any effect that light will
have on that work in a given time frame.
The following time frames assume proper mounting in an average setting
indoors. In Blue Woolscale it should be noted that each level is 3
times more exposure than the previous. The Woolscale equivalents to
ASTM classes is approximate only.
It is also easy to find differing interpretations of time for a given
total of light (measured in lux) The following is a good rule of thumb
and is based on MacEvoy's excellent 'The Worlds Finest Guide To
Watercolor Painting' and that in turn was based on Gottsegen 'the
Painters Handbook' and Colby 'A Suggested Exhibition Policy For Works
On Paper (Journal of the International Institute For Consevation:
Canadian Group 1992)'
ASTM l
Excellent. Equivalent of Blue Woolscale 7 and 8. More than 100 years
without change. (Probably more than 200 years in the case of Woolscale
8) (between 300 and 900 megalux)
ASTM l l
Very good. Blue Woolscale 6. Between 50 and 100 years without change
(100 megalux)
ASTM l l
l Fair. Blue Woolscale 4 and
5. Between 15 and 50 years without change. (10 to 32 megalux)
ASTM l V
Poor. Blue Woolscale 2 and 3. Between 2 and 15 years without change.
(1.3 to 3.6 megalux)
ASTM V
Very poor. Blue Woolscale 1. Less than 2 years without change. (.4
megalux)
Transparency Covering power
or glazing suitability
All you need here is a white surface with a black line several
millimeters wide. brush the test paint thinly across both the white and
the black. how well the color covers the black line is a good indicator
of relative transparency or opacity. Gouache can be tested in the same
way to ensure that the proportion of chalk in the formula will provide
adequate opacity.
Draw-downs Practical way to
examine strength etc
For this you will need a good quality house painter's scraper. It needs
to be one with a nice flexible blade about 75 or 100 mm wide (3 or 4
inches) You will also need white bond paper in sheets about A4 in size.
This test is especially useful for making accurate comparisons between
samples. A standard thing to do is always test against a paint of known
qualities. Place 2 dabs of paint at the edge of the scraper side by
side. then steadily and using a light pressure so that the paint goes
onto the paper in a thick layer. After 2 cm (1 in) increase pressure
and with the blade held near vertically draw the paint film out in a
thin layer. By holding the paper up to the light and looking through
the paint much will be revealed about purity (if you are checking a
bought tube) relative strength and so on. This test is likely to reveal
if a paint has been 'boosted' when testing manufacturers paints (that
is had a dye added to brighten the
color) and reveals the color's undertone. Dispersion problems should
also be visible in this test. Lightweight watercolor paper can be used
to test watercolors by wetting one half of the paper and drawing
the test from the dry to the wet area. As the paper is likely to be
thicker than cartridge paper a stronger light source may be necessary.
[ Go to Demonstration of making paint ]
References
Feller, R L, Artists Pigments 1986
(National Gallery Of Art / Cambridge University)
Gettens, R J, and Stout, G L, Painting
Materials: A Short Encyclopedia, 1942
(Dover)
Gottsegen, M D, A Manual Of Painting Materials And
Techniques, 1987 (Harper & Row)
Mayer, R, The Artists Handbook Of Materials And
Techniques, fifth edition 1991 (Faber
& Faber)
Roy, A Artist's Pigments: A Handbook Of
Their History And Characteristics, 1994
(Oxford University Press)
Various, Encyclopedia Britannica,
fifteenth edition 1981 (Encyclopedia Britannica, Inc)
Internet Resources |  Contact |
Frequently Asked Questions
|
|
|
|