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Fire Assay Furnace – Fusion

This 25 place fusion (assay) furnace is electrically powers by a 24000 Watt heating element allowing it to reach and hold a temperature of 1200 degrees C (2200 F).china manufacturer

With a working area of 56 cm deep X 59 cm wide X 19.5 cm tall, this furnace will hold:

  • 25 x 50 or 65 grams crucibles
  • 42 x 30, 40 or 55 grams crucibles


fusion furnace
The external structure of the furnace shell is made of galvanized steel sheets mounted on the bracket on the surface of the shell made of steel pipe. The blue paint; Furnace Body: the inner surface of the furnace is made of alumina refractory brick, and the back is insulated refractory brick; Heating mode and rated energy consumption; Temperature control mode: electric, automatic control, the highest temperature can reach 1200 degrees; Temperature sensor: “K” thermocouple, high measurement accuracy;

Power supply: the furnace body is equipped with a 10A, single-phase cable and plug, and the power load is 5A; Pneumatic control furnace door opening and closing, foot switch conveniently for users to operate, avoid high temperature and hot environment caused by security risks. fire assay furnace (2)assay-furnace-4-1assay-furnace-2

Structural Overview

This furnace has an easy to use main rotary on/off switch.

It is heated by 12 elements mounted in the side walls. It is ideally suited to continuous running as this will prolong the life of the elements.

Temperature is monitored and modulated automatically by a Type K thermocouple system and Omron temperature controller, pre-set for a maximum temperature of 1200 degrees Celsius. This controller is simple to operate, and has a 2-level display (PV and SV). that shows actual temperature as well as the temperature the furnace is set to.

The doors have Omron door switches installed that will automatically cut power to the heating elements when any door is opened.


Installation Procedure

Unpack furnace from crate and store in dry area. The furnace usually has timber supports installed inside for transport. Remove before operation.

2.1 Furnace requires minimum 1 meter space around it to carry out maintenance / repairs.

2.2 Install fuel supply. This must be carried out by a qualified technical person.

2.3 Air is required to run the pneumatics for door operation. Connect air supply to the filter on the furnace.

2.4 Electrical connection is 3 phase with Earth. Connection is made at the terminal strip inside electrical cabinet. Once connection is made, before turning on power, check all other connections as they may have come loose during transport.

2.5 Before starting please check whether all exhaust ducting is in place.

Installing Elements

3.1 When installing elements be careful not to force any into position.

3.2 The elements are fragile, and if there is too much pressure on them or if they hit the floor of the furnace they may break.

3.3 It is best to install the elements before the muffle chamber is in if possible, as then you can have two people to guide the elements through the bottom holes.

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3.4 When installing, make sure that the elements sit flat on the element holder underneath the body.

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3.5 Once all elements have been installed, wrap the element straps around the elements and secure with an Element Clip, making sure that you have a good connection.

Fusion Assay Furnace Element Clip

3.6 The lower end is connected to the power supply and clamped with a nut.

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3.7 Check that none of the straps and clips are touching the frame or the top cover.

3.8 Check that all nuts holding the connections (straps) are tight.

3.9 Install the top and side guards of the furnace.

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Element Connection

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Element connections are from front to back.

The first 4 elements are 1st or 2nd phase (depending on the left or right side of the furnace) and the rear 2 elements are part of the 3rd phase (on both sides).

Things About Elements

5.1 It is ok to have each phase on different voltages so that you can balance the amps. E.g. If one phase is only running at 5 amps then you can increase the voltage to that phase to run the same as the other phases.


5.2 Also if an element breaks or it fails for some reason, you must change all elements in that phase and then reset the voltage tap back to the setting required for new elements.

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5.3 If you only replace the damaged element then you will rapidly reduce the life of that element as you will still require higher voltage to operate the other 3 elements left in that phase. When an element has failed in a phase, most labs replace all the elements and then keep the undamaged elements to one side in case another element is damaged later. They can replace the older element into that phase without changing the voltage taps and thus save using new elements.

Silicon Carbide Elements

Generally a heating element made of silicon carbide is gradually oxidised due to its application over long periods, increasing its electrical resistance; that is, aging results. This is due to the partial oxidisation of SiC, which is formed into SiO2, diminishing the electrical conductivity of SiC according to the reaction.

SiC + 2O2 → SiO2 + CO2

To prevent oxidisation and to preclude an increase in electrical resistance, the surface is covered with a film coating, which, being stable without comparison, helps minimise aging and ensures a considerably longer life. When the electrical resistance has increased to about five times the incipient value, the service life of a heating element may be considered expired. In actual application its aging speed depends on the applicable temperature, watt density, operating type (continuous or intermittent), atmosphere in the furnace, and so on.

Effect of Applicable Temperature

The aging speed on SiC varies considerably according to its applicable temperature. The higher the temperature, the quicker the aging speed.

If however, the density of its surface load is diminished despite high temperatures inside the furnace, silicon carbide elements can be employed over a prolonged time.

Effect of Operating Type (Continuous or Intermittent)

Even when the furnace temperature and the watt density remain the same the effect on continuous operation of silicon carbide differs from that of intermittent operation (for instance operations only during the day): that is, the latter type has harmful influence upon silicon carbide. This is considered due to the fact that SiO2 formed by the oxidisation of SiC experiences abnormal expansion and contraction at about 250 degrees Celsius (the transformation point of cristobalite), the repetition of which imparts mechanical shocks to the inner structure of the heating element, producing tiny cracks.

Instruction on Replacing Procedures

When replacing the heating elements, replace all of the elements installed in the same regulating circuit. This mil not only ensure the long service life, but will also prove more economical for the following reasons:

Partial replacement of heating elements in a parallel connection mill overload the new elements, and that in a series connection will overload the old elements. This will not only disturb temperature distribution but also will impair the service life of the elements. Classify the removed elements according to the electrical resistance, and store them as spares.

As silicon carbide elements increase in resistance as they age the power supply to the elements is supplied per a multi-tapped transformer. The taps on the transformer fitted to the furnace may start as low as 30 volts and generally are in 5 to 10 volts steps to as high as 150 volts, with one terminal marked common or to volts.

To increase the voltage supply to the elements, move only the cables on the voltage taps.

Do not move the cable marked zero v (common) unless the transformer is fitted with 0v + 5v terminals, in this case the main taps are at 10v intervals and it is possible to split each 10v tap by moving the lead from the 0v tap to the + 5v tap then back to the 0v tap on the next change up on the main terminals and so on.

The resistance increase of the elements will be evident by a slowing down of the recovery time (heat up) of the furnace. This is also indicated by the ammeter/s.

The maximum recommended reading on the ammeter/s for this furnace is 25 amps.

It a possible that this reading may be exceeded by 10% upon stepping up the transformer. This is acceptable but higher readings will overload the elements and transformer.

Replacing Elements

Should one or more elements fail prematurely it is recommended that the complete set in that phase be replaced. The aged elements can be stored as spares and graded by their electrical resistance.

Partial replacement of a set of elements is NOT RECOMMENDED.

Element Life

As previously stated, the life expectancy of silicon carbide elements depends on several factors.

Because of the relatively low operating temperature of cupellation furnaces, the increase in resistance of the elements should remain within 7-10% per 1000 hours of operation.

After fitting a new set of elements, go back to the lowest lap on the transformer to start.

Starting Up Furnace

Turn on power. (If 415v check that electrician has included a neutral wire to the furnace. All the control equipment needs a neutral wire.)

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As you look at the furnace you will see 3 Amp meters. One Amp meter for each phase.

As start up the amps will slowly climb up to 45 amps (½ hour). If this does not happen after this time you will need to increase the voltage.

Isolate power then open the transformer access door.


Move your voltage leads (only) to the next voltage setting. You will see these clearly 90v, 100v etc.

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Optimum amps for operation of fusion furnace is 45 amps.

Once you have done this then slowly warm up the furnace to the cure the refractory.

Electrical Diagram

Fusion Assay Furnace Electrical Diagram

New Muffle Start-up from Cold Procedure

Regular maintenance is required to make sure furnace is running correctly.

When staring a furnace up with a new muffle installed or starting the furnace from cold you must do so slowly. This is to reduce any cracking in the cold muffle of furnace.

Muffle Start Up from Cold

Once muffle is in place and rear vent has been installed into furnace:

Start furnace and set to 250°C;

Run for a minimum of 2 hours;

Set temperature to 500 °C and run for another 1 hour;

Set temperature to 800 °C and run for another 1 hour;

Set temperature to 1200 °C and run for another 1 hour;

Set temperature to normal running conditions and muffle is ready for use.


To do this and to extend the life of your muffle and furnace, regularly check that muffle chamber has no cracks, if so then seal with some refractory cement.

Fusion Assay Furnace Cracks

Make sure a floor tile is always installed, with about 15mm of Bone Ash underneath it. Regularly scrape and clean the floor tile. If there is build-up on one side, flip the tile over to double its life. Change floor tile regularly.

Keep the floor areas around the furnace clean and free of dirt, as it will layer onto the transformer and control equipment. This chokes the equipment and makes it run hotter and it may over heat.

Remember a chamber is a consumable item in a furnace. You must change a chamber out at least 2-3 times a year if looked after correctly. If little or no maintenance is done on a furnace, the chamber can last as little as 1 month. If the chamber is left unchanged, the brick work of the furnace will be eaten away. This will severely reduce the life of your furnace.

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Failing to change Muffle when required.

Note the damage to the brick work.

Installing Muffle Chamber

Remove Thermocouple and lead extraction.

If replacing a muffle, remove old muffle and vent. Ensure that all debris is removed.

Before installing new muffle into furnace, you must make sure that the back wall back ledge and the front ledge are clean and smooth.

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Note: If the back wall, back ledge and front ledge are not cleaned properly, the muffle will not sit correctly, and will not last as long. Depending on size of muffle, lift in by hand or position with forklift.

Position new muffle in place and make sure it is pushed right to the back wall, and try to leave even gaps either side of the muffle.

Fusion Assay Furnace Muffle

Once muffle is in position, cut Lengths of 25mm Kaowool (20 mm wide x length of roll) and stuff around muffle. Make sure it is a tight fit and do not leave any gaps. Replace the thermocouple.

Fusion Assay Furnace Thermocouple

Once all kaowool is installed, smear a thin layer of mortar over the kaowool and lip of muffle.

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Installing Vent Tube

Fusion Assay Furnace Installing Vent Tube

Put a small amount of Mortar on the bottom of the Vent Tube. Once you have done this simply slide the vent in place.

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Installing Furnace Floor

This method can apply to all fusion and cupellation furnaces.

Reason: To stop the tile from sticking to the furnace or muffle floor so it can be easily removed when needs to be replaced.

If tile does not crack it is also possible to reuse the tile on the opposite side. Each time you replace a tile you must scrape out the old mabor powder or bone ash and replace with new.

Mabor powder is used to soak up any spills you have during fusion or cupellation processes.

The following is a tile install for a cupellation furnace.

Step 1: Shovel Mabor powder or similar product onto the floor.


Step 2: Using a flat edge, flatten off so that powder is about ½” thick. Make sure the service is level from front to back.

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Step 3: Place floor tile into position and settle down flat.

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Step 4: Sprinkle a little powder on top of tiles ready for the first pour. If a spillage occurs then scrape out to the front of the furnace and sprinkle powder back on the floor.

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By following this practice your tiles will last longer and the furnace will stay clean.


A good clean combustion chamber will allow the furnace to run efficiently. To do this and to extend the life of your muffle and furnace, regularly check that muffle chamber has no cracks, if so then seal with some refractory cement.

Make sure a floor tile is always installed, with about 15mm of Bone Ash underneath it. Keep the back Vent clean and clear, and change it when needed. Regularly scrape and clean the floor tile. If there is build up on one side, flip the tile over to double its life. Change floor tile regularly. Regular maintenance is required to make sure furnace is running correctly. Maintain Door ceramic blanket and change it if in poor condition.

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Door Repacking

The First Second and Third Layer: cut 3x30mm Kaowool wide (or 1x60mm & 1x30mm) wide and fill the inside of the door, making sure all 3 layers are tucked under the door’s lips.

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Fourth Layer: cut 30mm Kaowool so it fits on the inside lip of the door.


Final Layer: cut 25mm Kaowool to the length of the door plus 40mm (width=roll width) and place so that a 30mm overhang of kaowool is present on the edges shown.

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Using a guide like a screw driver, push up through the existing anchor holes in the door’s body, keeping the guide straight until it comes through top layer of kaowool. Once guide is through wiggle around to make a slightly bigger hole.

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Place a screwdriver through the hole (kaowool side up) then the cutter over the screwdriver (a piece of pipe with a circumference of about 25mm by 100mm long with a mark about 50mm up). Keeping the cutter at 90 deg, cut down to marked line in cutter (50mm) and pull out. Knock out kaowool stuck in cutter.

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Once cutter is pulled out there is a hole big enough to place anchor through.

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Screw anchors into holes cut out by cutter in kaowool until the bolts come through the pre drilled anchor holes in the door’s body. Once bolt is through secure with a wing nut.

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Once all anchors have been installed, use a straight edged, flat piece of wood and a sharp knife to cut off overhanging kaowool. Make sure the straight edge is lined up flush with the outer lip of furnace door top and bottom. Push straight edge down firmly and cut off excess kaowool.


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