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The production is fairly well
done, but is 'homegrown', so has no fancy digital titles and effects.
There are ambient noises like dive bombing hummingbirds and airplane noise,
though. Suppliers' list included.
The first video tape is 2 hrs
40 minutes. It covers the following:
measuring, marking and cutting
the frame steel
fastening the frame together, painting
measuring, cutting and placing the galvanized sheet steel
cutting and wrapping the fiber backup insulation in foil
cutting and placing 2 layers of brick
grooving the element bricks
measuring resistance of element, cutting, stretching to length, twisting
lead wires
placement of element in grooved brick sections
lid brick fitting and fastening
hinge fastening
crucible placement and brick cutting
electrical connections
lid safety switch mounting
thermocouple selection and placement
The second tape includes:
controller component options
basic start-up and shut down procedures
crucible care
routine maintenance checks
element replacement
other maintenance considerations
refitting one after 2 years intense use
building a digital controller for under $200
The furnace takes about 6 hours
to get up to 2300 from a cold start, charged. When hot, it takes
a couple hours to melt additional full charges. Cooking and squeezing
schedules for attaining good clear glass would be similar to other furnaces.
The 15 pound capacity electric
glass furnace is a square box, 20 inches by 20 inches. It has 7
inch legs to provide ventilation below. It has a 9 inch square gathering
door on the top, so that the gathers are taken from on top of the glass
supply, instead of from the side. The gathering door sits on an
18 inch square lid, for maintenance access.
The frame is constructed of
fairly light angle iron, with galvanized sheet steel outer liner.
It has 2 layers of K-23 insulating fire brick and one layer of 1 inch
mineral fiber board as backup insulation before the outer steel skin.
The pyrometer and controller
options affect the price of the units. I prefer a programmable digital
controller, mercury relay, and R-type alumina-sheathed thermocouple.
I have used the K-type thermocouples in this furnace, and they last 2
months max before they burn out. The K-types are OK for pottery
kilns because they are only at 2000-2200 f for a very short time with
each firing. The electric glass furnace requires that the temperature
be 2200-2300 f for several hours a day. The K-type thermocouples
are under $35, and the R-types in a protection tube are $200. Infinite
control switches can also be used with this unit, but it is not recomended.
(These are the small controls that have knobs and are used in electric
kitchen stoves.) This is because there is no communication between
the device setting the temperature and the actual temperature in the furnace.
The infinite control switches are under $35, the Dwyer process controls
are $250. The last option would be the relay that switches the current
to the element. A standard industrial contactor rated at 40A can
be found for $20. Mercury relays are silent, reliable, completely
sealed, and are guaranteed for millions of cycles. They are about
$70 for a 60A rated 2-pole unit.
There is a commercial electric
crucible furnace on the market right now which is basically a pottery
kiln with a pot sitting inside it. It includes a K-type thermocouple
and infinite control switch. These would be OK for occasional hobby
use, but would not last very long in a production shop.
The crucible in this furnace
sits below the bottom of the top brick layer. The top lip of the
crucible does not come up to the bottom side of the gathering lid.
The entire height of the crucible is surrounded by element coils, so it
heats up right to the top.
The element is 14 gauge Kanthal
A-1 wire, wound to be 3/8" outside diameter. I use 22 ohms worth
of this element in this particular furnace because I run it straight up
from cold and don't want to crack the crucible. If the element was sized
down to about 16 ohms, it would get hotter faster, but wouldn't last as
long.
Advantages:
silent operation
portability
top-gathered and top-charged
modular brick system for easy (1-1/2 hr) element replacement
low power consumption
crucibles are $70
elements are $50 or less
top entry means easy visual inspection of glass level and quality
top entry makes for easy spot color melting on the surface
6 inch round entry hole means pipes and rods don't get too hot
Disadvantages:
sloppy gathers can result in erosion of brick around the entrance (same
with most furnaces)
elements need replacing after every 80 or 90 "hot" days (same with all
electric furnaces which use radiant heat coils)
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