When is kiln lid still worth repairing?
A lid can last the life of the kiln if you are as gentle with the lid
as you are with the greenware you load into the kiln. Do not entrust
anyone
else with loading your kiln until they understand the importance of
careful
handling.
An ounce of prevention
On kilns with a lock-in support, the most common cause of lid damage is
pulling the lid downward before disengaging the support. This can crack
the
lid around the hinge the very first time. A kiln hinge has never been
designed that will withstand this type of stress. Lift the lid slightly
before disengaging the lid support.
On kilns with chain supports, the most common cause of lid damage is
allowing the lid to fall backward against the chains from a vertical
position. Once you raise the lid straight up, do not let go until it
rests
against the chains. Lean it back slowly and gently. Never place
anything
on top of the lid, such as greenware or kiln shelves. The larger the
kiln
the easier the lid can crack. Keep the lid of a hot kiln closed until
the kiln has cooled to room temperature. Opening the lid while the kiln
is
hot cracks the inner lid surface. This also crazes ceramic glaze loaded
on
the top kiln shelf.
It is common to raise the lid of a glass fusing kiln to cool the glass
through the devitrification range. We would suggest opening it only a
couple
of inches to minimize temperature shock to the lid. Thermal shock,
however
is usually not as sever in a glass fusing kiln as a ceramic one because
fusing temperatures are lower than ceramic.
When not to repair a lid
Too heavy a wash coat of cement on the lid surface will eventually
leave
the lid looking like a cracked windshield. This is because the cement
has
a different rate of expansion than the brick. This type of cracking,
however, is only surface damage and is not serious. If the coating
peels,
merely scrape it off with grit cloth and re-coat.
It is best to repair a cracked lid only when the crack is contained to
one small section, such as the hinge, lid handle or lid support.
If the crack runs across the lid over a wide area, it is usually best
to
leave the crack alone. Most lids with such cracks are still usable the
way
they are.
Nine times out of ten, a heavily cracked lid crumbles when separated at
the cracks. it usually falls apart when you remove the band. When you
separate one crack, the lid may well separate at the other cracks that
were
invisible before.
Repair only a cracked section that you can saw off with one straight
cut. Saw the section clean through the lid.
It is usually not advisable to cut a hole into the lid and cement a
brick
piece into it. This is because it would be difficult to get a really
tight
fit, which is essential. If the crack ran through the middle of the
lid, you
would have to saw out the entire crack and replace it with fresh brick.
Such
a crack would probably not be worth the expense of fixing.
Should you tighten the band on a heavily cracked lid?
It is probably not worth your time. All around the lid, the band
fastens
to metal retainers inside the firebrick. The lid retainers keep the
band
from slipping off the lid. To tighten the band, you would have to
remove all
the screws holding the band to the retainers and then drill new holes
after
tightening the band. This would buy you only a little more service from
the
lid anyway.
You can fire a cracked lid, even one cracked like a windshield. We have
seen kilns with lids that were so cracked they bent slightly when
lifted, and
they were still firing. The lid expands when heated, closing the cracks.
Heavily cracked lids drop dust inside the kiln. Before each firing,
vacuum the dust from the inner lid surface.
Replace the lid when it becomes so cracked that the dust is
unavoidable. If
you fire only greenware in that kiln, however, the dust may not be a
problem.
Can you separate the halves of a crack, apply cement, and press them
back
together?
This is rather difficult. It is much easier to cement two flat surfaces
together than matching jagged surfaces. The reason is that if you do
not
press the halves back together tightly, the cement that fills the crack
will
be too thick. This thick layer of cement will fall out later since it
expands and contracts at a different rate than the firebrick. If you
can
press the cracked halves back tighter very tightly, then it is possible
to
cement two jagged matching surfaces. But don't be surprised if it just
breaks again in the same place due to the difference in coefficiency of
thermal expansion.
How to repair a cracked lid
1) Remove the lid bank by first removing all screws from it and then
loosening the band.
2) Trace the damaged area of the lid onto a piece of paper.
3) Saw the damaged section of brick in one straight line. Use a band
saw or
hack saw. Smooth with sand paper.
4) Cement together a new brick section and let dry overnight. then
match
the new section to the lid by sliding together back an forth on a flat
surface. Sanding also helps. Both surfaces must match perfectly.
5) Use an air hose, soft brush, or vacuum cleaner to remove dust from
the
surfaces to be cemented. All dust must be removed.
6) Apply cement to one of the two brick surfaces. Press surfaces
tighter
quickly. then slide back and forth to work cement into the brick pores
and
work out excess cement. Stop when you feel resistance. the seam between
the
lid and new brick section must be very thin. Scrape off excess cement.
7) Let dry overnight. Cut out the brick section using the paper tracing
you
made in step 2. Then sand away excess cement from lid surface.
8) Use a hacksaw to make a groove in the new lid section for lid
retainer (if
necessary). Reinstall the lid band.
9) Clean off all dust with a whisk broom and apply a very thin wash
coat of
cement. With a soft cloth, immediately remove all of the cement coating
possible.
After the kiln is fired, it may seem that the thin wash coat has
disappeared. However, you have actually left the lid with a hard, dust
proof
coating.
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