Crucible furnace
Crucible furnace
Type label
An old crucible furnace, type "K 59" from a Czech company "ESA Elektrodruzstvo Praha", vintage about 1970, was obtained for an affordable price. The top lid was missing.
Crucible furnace | Crucible furnace | Type label |
The furnace has a roughly cylindrical shape, with height 246 mm and outer diameter 235 mm. It is fairly heavy. It is rated for temperature up to 1000 °C, and rated electrical power of 750 W at 220V (equals 900 W at 240V).
The diameter of the crucible chamber is 97 mm, its depth is 92 mm. Its walls are virtually vertical. The bottom is flat. The height of the ceramic rim, included in the chamber depth, is 7.5 mm.
The height of the insulated heated part is 151 mm, the "feet" underneath (with the power socket and wiring) are 95 mm.
The width of the top ring, between the outer rim and the crucible space edge, is 69 mm. The depth of insulation under the crucible chamber is calculated to be about 59 mm.
The device has an ancient kind of high temperature power socket with a ceramic plug.
 Power socket |  Switch |  Switch |  Furnace top |
 Furnace side |
The wiring, at the bottom of the furnace, is covered with a perforated sheetmetal. The cover is attached with two M3 screws.
 Furnace bottom |  Furnace bottom |  Furnace bottom |  Furnace bottom |
 Bottom cover |  Bottom cover |
There are two heating spirals, with one common end. Their terminals are ceramic-insulated binding posts with brass screws and nuts.
The spirals have cold resistance of 53 and 15 ohms; the latter is shorted with a switch, giving the choice of 68 ohm (711 watts at 220V, 847 watts at 240V) or 53 ohm (913 watts at 220V, 1086 watts at 240V). The real resistance will be somewhat higher, as the wires become red hot.
 Schematics |
There is no feedback regulation. The furnace has two power level, I and II. It has no master power switch, the heat is on always when it is connected to the mains. The switch shorts one of the spirals, increasing the current and the released heat. The rated maximum time of this higher-power operation is 70 minutes. The maximum achievable temperature is said to be 1000 °C with the original (missing) lid. With a small ceramic cover from a water bath the temperature is said to be a hundred degrees lower.
 Electrical wiring |  Switch wiring |  Socket and heater wiring |  Heater coil resistances |
The top side is secured with a metal ring, held in place with four self-tapping screws. After removal, access was gained to the top of the furnace. The top is made of a flat round ceramic tile, over time cracked to three parts. The bottom of the ceramic plate is ridged, to match the underlying structures.
Underneath the top tile there is a ceramic crucible, with integrated heating spirals. This crucible is set in another crucible. The space between, and between the outer crucible and the metal can, is filled with ceramic insulation potting similar to cement or plaster. It is fragile and releases sandy material and dust when handled. The dust mildly irritates skin; washing hands afterwards is recommended.
 Top ring removed |  Top ring |  Without top ring |  Without top ring |
 Bottom sides of ceramic top |  Without top ceramics |  Without top ceramics |  Crucible chamber |
 Resistive wire detail |