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TECHNICAL^SUPPLY DEPARTMENT
William Braid White, Technical Editor
An Investigation of the Causes for
. Checking or Cracking in Piano Finishes
Most Factors Controlling the Durability of a Finish Beyond the Control of the Manu-
facturer of Finishing Material—Some of the Causes and Remedies
AIR lines, checks, and cracks are so
familiar in finishes on furniture, pianos,
automobiles, and many other objects
that there is little need for a definition or de-
scription of them. Everyone knows a checked
finish when it is seen. Hair lines, checks and
cracks are terms used mostly to describe the
degree of deterioration; when the deterioration
has just started the defects are usually small
and are referred to as hair lines; in the more
advanced stages these hair lines become larger
and more pronounced, running into each other
so as to form a checkered effect and are then
known as checks. When the checks open up
quite wide or the hair lines run in more or less
straight lines that are quite visible the defect is
referred to as cracking. "Alligatoring," "crazing"
and similar terms are also used at times to de-
scribe these defects. All of them usually have
an origin that can be traced back to a few
causes.
It is doubtful if the time will ever come when
absolutely all troubles of this kind are elimi-
nated in finishes. There are many factors that
exert an influence over the durability of a fin-
ish, over which the paint, varnish or lacquer
manufacturer has no control. For example, all
the materials used may be of the very best
quality, yet the finish will not be permanent if
the surface on which they are applied has not
been properly prepared to receive the finish or
if the various coats are not properly applied and
dried. The surface, whether of wood or metal,
must be clear and free from moisture, oil,
grease, dirt, rust and other defects. Wooden
surfaces must have the proper moisture con-
tent, neither too dry nor too wet, otherwise the
subsequent changes of moisture content may
spoil the finish.
Even the best of finishing materials and the
best of workmanship may still fail to give sat-
isfaction in the life of the finish if the ultimate
user of the finished article either purposely or
unknowingly submits the finish to conditions
which promote rapid deterioration. All of these
arc conditions over which the maker of the
finishing materials has no control and it will
be seen that a satisfactory finish depends as
much upon the conditions under which tin
materials are applied, upon the methods of ap-
plication, and upon the treatment of the finish
by the ultimate owner, as upon the quality of
the materials themselves, says a recent article
in Veneers.
The most frequent cause of checking or crack-
ing lies in the soft or not thoroughly dried
undercoats. Even when the utmost care is
used in preparing the surface and in applyinp
the coats, the danger is not entirely eliminated
unlej-s each coat is at least as elastic as the pre-
ceding ones and unless each coat is thoroughly
dried all the way through and not merely on
top.
It is a well-known fact that all paint and var-
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nish materials dry by a combination of the
evaporation of the solvent portions and oxida-
tion of the non-volatile oils and gums. Within
a short time after the material has been applied
the volatile thinners have evaporated to a great
extent and the finish becomes sticky or tacky.
From that point on, the drying proceeds by the
oils and gums absorbing oxygen from the at-
mosphere and thus being converted from a
liquid to a solid form.
Most paint and varnish materials have a ten-
dency to dry from the top down, that is, they
will harden on top within a few hours and yet
be more or less soft underneath, requiring con-
siderable time before they are hardened through
the entire thickness of the coating. If another
coat of finishing material is applied before the
oxidation of the previous coat has proceeded
far enough, the new coat begins to harden from
the top down and soon forms a dense layer
which prevents the oxygen from easily getting
to the bottom of the first coat of only partially
dried material. If the last coat stands long
enough it will harden through and yet the bot-
tom portion of the first coat may not be thor-
oughly dried. If the finish is subjected to a
change in temperature, or if the amount of
moisture in the wood is increased or decreased
there will be an expansion or contraction of
the wood and the finishing, material will have to
contract or expand also. Now the portion of
the finish that is not thoroughly dried will ex-
pand or contract over a greater distance, with
a given change in temperature, than will the
hardened top film without cracking. As a re-
sult the elastic limit of the top film may be ex-
ceeded and checks or cracks will appear.
Regardless of whether any great changes in
temperature or moisture content take place,
there is danger of checking or cracking develop-
ing under the above conditions, due solely to
the fact that as the undried portion of the film
becomes dry it will change in volume. Since
the top film is hard and already under a strain,
similar to that of a rubber band drawn tightly
over a wooden surface, it may not be elastic-
enough to change shape with the underneath
portion of the film and will be broken in pieces
;is the underneath portion chances in volume.
EASY TO REPAIR
DAMAGED FINISHES
OU can easily repair the damage
to varnished surfaces through fol-
lowing the instructions given in our
book "How to Repair Damage to Var-
nished Surfaces." Dealers everywhere
are enthusiastic over the ease with
which this is done—and the losses
which have been eliminated. A copy
will be sent to you free upon request.
Y
The M. L. Campbell Co.
1OOS W. sth St.
Kansas City, Mo.
George W. BraunsdorS, Inc.
Direct Manufacturers of
TUNERS' TRADE SOLICITED
William Braid White
Associate, American Society of Mechanical
Engineers; Chairman, Wood Industries
Division, A. S. M. E.; Member, American
Physical Society; Member, National Piano
Technicians' Association.
Consulting Engineer to
the Piano Industry
Tonally and Mechanically Correct Scales
Tonal and Technical Surreys of Product
Tonal Betterment Work in Factories
References
to manufacturer! of unquestioned
position in industry
For particulars,
address
209 South State Street, CHICAGO
Piano Tuners
Also—Felts and
Cloths, Furnished
In Any Quantity
Woodside, L. I., N. Y.
27
Any condition which causes a portion of the.
undercoats to be softer or more elastic than
the top coats, usually results in cracking. Im-
properly dried coats will cause cracking, as men-
tioned above. If the coats are heavier in some
spots than others, much the same effect will be
produced in those same spots. If the top coat
of varnish or paint contains less oil or is
"shorter in oil" or less elastic than the bottom
coats, exactly the same condition exists.
Cracking or checking can be traced, many
times, to a desire to apply the finishing mate-
rial too heavily. It is better, from the durability
standpoint, to apply three thin coats of mate-
rial than to put on two heavy ones. Where a
heater is used on the spray booth for warming
the varnish before it is applied there is often
:i tendency to apply the varnish too heavily. In
either case, the varnish is likely not to harden
through properly and as it drie^ out thoroughly
later on may develop checking and cracking. A
lack of proper drying conditions, such as the
proper temperature and relative humidity, may
prevent the coating from hardening through and
give trouble later.
Improperly dried veneer is sometimes found
to be the cause of checking or cracking. If the
veneer is too dry at the time it is finished it may
absorb more moisture from the air after the
finished article is put in use. On the other
hand, if the veneer is not dry enough at the
time of finishing it will lose some of its mois-
ture later. In either case there will be a change
in relative volume with the change in moisture
content and this change may be sufficient to
disrupt the hardened finish and cause checking
or cracking. Likewise unprotected edges of
wooden panels may absorb enough moisture
during the rubbing process to cause checking,
or the wood may take on moisture from damp
cellars or other sources through unprotected
edges of the wood, after the finished article is
in the home. In this case, however, the check-
ing or cracking usually extends only a short
distance back from the unprotected edges and
the center of the panels may remain unchanged.
Regardless of how good the finishing material
may be or how carefully it is applied and dried,
hair lines are often caused by sudden changes'
in temperature. The outer film of the finish
becomes cold first and consequently shrinks in
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