Knowing what has happened to your piece when it is broken
or cracked is important to developing your skills as a kilnformer. Most of the knowledge about diagnosis comes
from looking carefully at the cracks and the shapes apparent in the flawed piece.
Breaks
in the Kiln
Breaks in fusing at tack or full fusing levels in
the kiln are generally of four kinds.
Breaks
with hooked ends
Breaks that go across the whole piece, with a hook or
significant curve at each end, usually indicate an annealing problem. The
slight hook seems to result from inadequate annealing. The break will have
sharp edges as it occurs as the glass is entering the brittle stage.
Multiple
breaks in a crazed pattern
Crazed glass – similar to the cracks in ceramic glazes -
usually indicates the glass has stuck to the supporting materials. These
materials can be shelves or moulds. It is a sign there was not enough separator
present between the two surfaces.
Breaks
following the edge of glass pieces
Breaks that skirt around colours or pieces of glass almost
always indicate a compatibility problem with the glass pieces chosen. In severe cases the crack will be all around
the incompatible pieces of glass as though it is trying to escape the base
layer. Sometimes the break will be from
side to side, but skirting the incompatible glass. These breaks will have sharp edges as the
compatibility problem only becomes apparent on the cool.
Breaks from side to side following the line of glass
pieces is not an infallible indicator of incompatibility, though. Glass which has varying levels or thicknesses
can break alongside the thicker areas, even though the glass is compatible. Often
the break will be rounded due to temperature differentials in the glass on the
heat up. As the glass continues to get
hotter, glass pieces on top - or strongly contrasting colours - can heat as
such different rates that the stress overcomes the strength of the glass.
Of course, this kind of break can be sharp because the
break occurred during cooling. In
effect, this appears to be an annealing problem when it really is a problem in
matching the scheduling with the annealing requirements of a complex
piece. You need much longer soaks and
slower cooling on tack fused pieces than on flat fused ones.
These two contrasting causes of a break means that you
need to think about how the glass is layered.
One is to do with compatibility and the other to inadequate annealing
due to the complexities of the layup.
They also tie up with the fourth cause of breaks.
Breaks can also follow the edges of inclusions. This of course, indicates
incompatibility. All metals are
incompatible, but if thin and not excessively large in relation to the piece,
the glass is strong enough to contain the stress. When the metal or other inclusion is too
large, strong, or thick, the glass will break or show cracks around the
inclusions.
Broken
and separated lower layers
Sometimes people will open the kiln to find the lower
layer of a multi-layer piece has broken and separated a small distance. This is the fourth kind of break. This break will
most often be a nearly straight break from edge to edge. The broken edge will be rounded but the top
layer(s) will have the expected profile. This is an indication that the heat up was too
fast not allowing the lower layer to achieve the same temperature as the
top.
This most often happens where there is an exposed lower
layer (which gets hot) along with areas on top that get equally hot, but not the
glass underneath. Glass is a poor
conductor of heat, so the upper layers "shade" the heat from the glass
below. The temperature difference
between the two can be great enough to break the base glass apart but leave the
top intact. You know this was on the
heat up because the layers of glass could move independently when the base
broke and moved under the upper layers.
The glass was not hot enough to be sticky yet, so it had not reached
lamination temperatures before the break.
Rounded
vs. sharp edges
In addition to the location of the breaks, the condition
of the edges is important in diagnosis of the cause of the problem. The
accepted rule is that rounded edges mean the break occurred during the heat
up. Sharp edges occur during the
cooling. This is most often the case
(but see the conditions for slumping). For flat pieces breaks that occur on the
heat up will be rounded to some extent.
In a full fuse, usually the edges of the break will be rounded similar
to the outside edge.
Cracks on the bottom surface
Sometimes the broken pieces will recombine either
partially or all along the line. There
may even be a full recombination leaving only a crack like appearance on the
bottom. This recombination also will be the case where there was where only a partial break or crack in the early
stages of firing. It leaves a smooth top surface, but a visible crack on the bottom. That means
there is only a marginal reduction required in the scheduling of the initial rate
of advance, as the temperature differentials were not great enough to break the
piece completely across.
Force of Breaks
The space between the broken pieces shows the relative
force that caused the break. Greater
space is related to more stress; lesser space or only partial cracks indicate a
lower amount of stress. The amount of space indicates the degree of change required in
scheduling. A small parting of the glass requires only a little (maybe 10% - 15%) reduction in the rate of advance. Large spaces indicate that much slower rates of advance are required, and possibly a complete rethink in the scheduling of the firing.
Slumping
breaks
Breaks in slumps are usually caused by a too rapid rate
of advance. But this is not always the case.
The usual check of a sharp or rounded edge to tell when the break
occurred does not work well at slumping temperatures. The edge will be sharp whether it occurred on
the heat up or the cool down because the temperature is not high enough to significantly round the edges. The test must
be different on slumps than that of sharp edges. The test is related to the shape of the pieces. Take the pieces out of the mould. If you can fit them together exactly, the break occurred on the cool
down. This usually will mean the anneal
soak was too short and the anneal cool too fast.
Most slumping breaks occur on the advance in temperature.
The means of determining when the break
occurred can be tested by putting the broken pieces together. If they do not match exactly, the break
occurred during the heat up. This is
based on the observation that broken pieces separated slightly in the mould by
the force of the break on the heat up, and so will slump in the mould in slightly different
ways from each other due to their positions.
Remember the blank for slumping is thicker than the original
un-fused pieces. This thickness requires a slower
heat up than the original blank consisting of separate pieces. In addition, the glass is supported at the
edges of the mould which can allow the central area of the glass to heat faster
than the edges, so further slowing the rate of advance is required. These two factors of thickness and supports explain most of the breaks
during slumping.
Splits
in slumps
Sometimes the upper surface of the slump appears
fine. It is the bottom that exhibits a
split or tear that does not go all the way to the upper surface of the glass. This is similar to the cracks on the bottom of a flat piece described above. It indicates the rate of advance was too - but only just - too fast. The rate of advance has been quick enough to get the top heated and become plastic. But the lower surface is still
cold enough that it is brittle. The weight of the upper softened glass begins
to push down before the bottom has become hot enough to be plastic. The force of the weight of the upper portion of the glass can be enough to cause the glass to separate because it is brittle, rather than move as the surface does. This split on the bottom but not the top indicates a slightly slower rate of
advance for the thickness of the glass is required.
Breaks
out of the Kiln
Breaks
after the piece is cool
Breaks that occur days, weeks, months after a piece is
cool can be impact damage, annealing or compatibility problems.
Impacts
Impact breaks will be obvious in handling or moving other
pieces near to the affected piece. Usually there is evidence of impact by a small chip removed from the glass at the origin. The
piece may or may not have been stressed to allow an easy break rather than a
chip. It is not possible to be sure of the
secondary cause after the primary impact damage has occurred.
Breaks
in warm glass
If the break occurs shortly after having been removed
from the warm kiln, it is probable that the thermal shock to the glass has a
contributory factor to incompatibility or inadequate annealing. The diagnosis of the cause is the same as for
breaks in the kiln - hooked for annealing and straight or following colours or
inclusions for compatibility.
Breaks
in cold glass
If the glass has been sitting undisturbed in a shaded
place and suddenly breaks, the reason can be there was an incompatibility or
that the annealing was inadequate. There
usually is not much difference in the breaks in a piece that has been cold for
a long time. If the break distinctly
follows colours or pieces of glass, that would indicate a compatibility
problem. If the break crosses colours
and thicknesses it is more likely to be an annealing issue. But, as you can see, there is no certainty
in this distinction as to the causes of breaks a considerable time after
removing from the kiln.
Glass
in strong light
Glass placed in strong sunlight that breaks can be
incompatibility or simply contrasting colours being heated unevenly by the
sunlight. It is difficult to tell with
certainty whether it is compatibility, annealing, or heat differentials that
have caused the breakage.
Problem
Solving
The essential purpose of problem solving is to prevent the same thing
happening again. To solve the breakage problem, you need to think about the
interrelationships between the various parameters – firing rates, soaks,
cooling rates; and the ways in which the glass was set up.
Rounded
edges
If the break is shown to be in the early stages of the
firing, they most generally are caused by thermal shock. They will generally be straight on an evenly
thick piece. If the piece is with
variations in thicknesses, the line of the break may follow the thicker pieces.
In both cases, you need to think about the rates of advance you are using. If the separation of the edges is small
enough that they have begun to recombine later in the firing, the rate of
advance was only a little too fast. If
there is considerable space – say more than a finger width – the rate of
advance was significantly too fast.
Sometimes the condition of the upper glass can give an
indication of when in the firing the break occurred. On a first firing, if the upper piece has broken
together with the lower one, the break occurred after the pieces became sticky.
This would mean the break occurred at or higher than laminating
temperatures. This is rare during the heat up.
If the break has moved small top pieces, it indicates the
break occurred early in the heat up.
Sometimes the break will occur under the top piece. Later it slumps and fuses into the space created by the break. This also indicates a break early in the
firing. All these conditions indicate that the initial
rate of advance needs to be slowed to avoid the thermal shock. It does not indicate that soaks should be
added at various stages up to the softening point of the glass. Glass generally behaves better with steady,
gradual inputs of heat rather than quick rises with soaks (although there are exceptions).
Sharp
edged breaks
These occur generally on the cool down or after the piece
is out of the kiln for a while. If the
break has occurred in the kiln, you should look at it carefully before moving
it. The relative location of the pieces
can tell you some things about why.
Crazed glass normally indicates the glass has stuck to
the supporting material – shelf, moulds, or other rigid materials. This crazing may all still be in one piece,
or slightly separated, sharp edged chunks.
These effects indicate there was not enough, or appropriate, separator for the process used.
The distinction between annealing and compatibility breaks is given above.
Breaks all around a piece or pieces – looking as though
they were trying to escape the base - clearly indicate an incompatibility
problem. You need to identify that glass
and separate your stock of it from the rest of your fusing glass.
Cracks that skirt pieces of glass can be
incompatibility. This is easiest to
determine on flat pieces which have been full fused, or nearly so. There is not a variation in thickness to
complicate matters. In full fusing, if
the break skirts around a piece or pieces of glass along its path, it is likely
caused by incompatibility between pieces and their base.
Breaks skirting pieces can also indicate problems with
thickness, especially in tack fusing.
The more angular the tack fusing is, or the greater the difference in
thickness, the greater the potential for an annealing break. The annealing soak for tack fusing needs to
be significantly longer than for a flat fused piece of even thickness. Recommendations vary, but the anneal soak time needs to
be at least twice the thickest part. The anneal cool rate also needs to be half that for the the thickest area.
Breaks or cracks across the piece with hooked ends
indicate inadequate annealing. This will
require some consideration to come to the appropriate length of soak and rate
of the anneal cooling. The anneal soak is about getting all the glass to the same temperature - top to bottom, side to side. The soak is about temperature equalisation not just annealing. This is shown by the Bullseye research on annealing thick slabs. They discovered that a longer soak at a lower temperature can provide as good a base for the anneal cool as a higher temperature. The differences are that the soak at the annealing point can be shorter, but the annealing cool is much longer.
Annealing continues below the anneal soak - whether you chose the annealing point or a temperature below. Bullseye uses a temperature about 30C below the annealing point. This can apply to any glass. Because the glass is cooler, a longer temperature equalisation soak is needed. But the anneal cooling range is shorter, making for a reduction in cooling time for thick slabs.
The point of this discussion is that when considering the solution to annealing breaks, you need to have a relation between the temperature equalisation soak and the rate of the anneal cooling. If you have decided you need a longer soak, then you also need to reduce the rate of the anneal cool. If you do not, you will still have annealing breaks or even thermal shock breaks, even with long soaks at or below the annealing point.
Breaks
of slumped pieces
Breaks in slumping almost always appear to be sharp
edged, unless you look carefully at the edge.
Fitting the pieces back together will give an indication of when the
break happened. If they fit, the break
occurred upon cooling. The anneal may
have been inadequate, or the cooling too fast.
Unfortunately, in a formed piece, the curved hook of an inadequately
annealed piece does not often show up.
If the break occurred early in the firing, the piece may
still have sharp edges, unless you were firing at the upper end of the slumping
range. Here again the test of trying to
put all the pieces back together is important.
If the pieces do not fit exactly
together, the break occurred during the heat up. This will mean that you need to slow the rate
of advance for subsequent pieces.
“It
hasn’t happened before” Scenario
Often people experience breaks even though the set up was
very similar and the schedule was the same over several pieces. There are two responses to this – “what did
you change for the firing of this piece that broke”, and “you have been skating
on the edge of disaster for a while.” Glass behaviour is predictable. Since the
break occurred when the setup was very similar, and the schedule was the same,
something has changed.
The first
thing to do is to test for stress. This means test before the piece is broken,
as once the piece has broken most, if not all, the stress has been
relieved. You will need to construct
another piece in the same way as the successful or the broken one – whichever
you prefer. Test the flat fired piece for stress. Remember to include an annealing test, so you can
determine if the stress is compatibility or annealing related. If there is stress in the flat piece, but not
in the annealing test, you need to consider whether all the glass is
compatible, or you need to slow the annealing cool for the larger test piece.
Next you
need to consider what was different.
Review the differences in set up of the piece – colours, arrangement,
thickness, volume of material used – everything that might be different at each
stage of the layup. Note these
differences and review them one by one.
Could have any one element been sufficient to make the firing conditions
different? Could a combination of these
differences have been significant?
Are there any differences in the firing schedule? Have you made any little tweaks in the
schedule? What is different? Different
times of the day, different power supply, plugs in or out, venting, peeking,
different shelves (or none) – any small thing that could have introduced a
variable in the firing conditions.
Conclusion
Although breaks generally have only three causes –
thermal shock, incompatibility and inadequate annealing – the diagnosis of
which it is and how it was promoted is complex.
All three are forms of stress. To
problem solve, first attempt to determine the type of stress that induced the
break. Then attempt to determine the
cause of that stress.
It is important in the early stages of a new kind of
piece, or early in your fusing career to test for stress after each firing
(although I fail in this often). This
will give you the information to progress to the next firing or to revise the
conditions – glass or schedules – to remove the stress for this or subsequent
pieces.
