Robert Coenraads Geoffrey Sobels Early prototype box- shaped, - - PowerPoint PPT Presentation
Traditional Thai Coke Furnace Design and Analysis Robert Coenraads Geoffrey Sobels Early prototype box- shaped, coke-fired gemstone heat treatment furnace located at the Inverell laboratory of Mr. Geoffrey Sobels. The air blower fan is
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Early prototype box- shaped, coke-fired gemstone heat treatment furnace located at the Inverell laboratory of Mr. Geoffrey Sobels. The air blower fan is mounted on the base of the combustion chamber and blows air directly upwards into the combustion
flame visible at the top
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Detail of the box- shaped combustion chamber showing the thermocouple and wire sticking out of the right hand side of the furnace between the lid and the body of the furnace. The white material is the fireproof wool lining the inside of the furnace. It was later decided that a cylinder-shaped furnace would give a more even heat distribution within the furnace
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Typical 3 storey terrace dwelling in Bangkok, Thailand. The Gemsellers gemstone heat treatment and cutting factory was located in this building with the office on the ground floor, the cutting factory on the first floor, manager residence on the second floor and the heat treatment facility located on the rooftop.
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Mr Geoffrey Sobels and Mr. Kittisak Manepong in the ground floor
Specifications for the furnaces designed by Mr. Geoffrey Sobels and Dr. Robert Coenraads.
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Two of the furnaces designed by Dr. R. Coenraads and Mr.
are the “Short Boy” furnace (left) with a 26 litre combustion chamber and the “Baby Boy” furnace (right) with a 3.25 litre combustion chamber.
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The “Tall Boy” furnace (left) has a 52 litre combustion chamber created by the addition
the Short Boy furnace. The 3.25 litre “Baby Boy” furnace is shown at right for comparison.
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The furnaces were constructed from 6mm rolled steel plate in one of Thailand’s numerous machine shops. Being finished here are the “Tall Boy” extension tube and the “Baby Boy” furnace.
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The finished “Baby Boy” 3.25 litre furnace, being held by one of the engineers, illustrates the light weight and transportability of this model.
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Themal insulation blanket (called “Kao Wool”) is purchaed by the roll. It is cut into the correct size and then used to line the interior of the combustion chamber and lid of the furnace. Care must be taken not to breathe the fibres
heated.
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Mr Geoffrey Sobels and Mr Kittisak Maneepong select rubies from Tanzania and sapphires from Inverell, Australia for a number of heat treatment experiments. In order to evaluate the benefits of any particular process, a sample of the untreated material must always be kept for later comparison with the final product.
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The heat treatment process begins with a thorough cleaning of the stones. Any extraneous oxides or dirt on the surface may affect the final outcome (i.e. colour and appearance) of the stone. Very powerful and corrosive cleaning agents are used. In this case, Mr Kittisak Maneepong is using hydrofluoric acid in the area behind the factory. This is not the correct manner in which hydrofluoric acid should be handled!
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Different chemicals used for corundum heat treatments are shown here. Chemicals are used to achieve the following outcomes: to modify the atmosphere of the sealed gemstone crucible; as a medium to allow the diffusion of ions into and out of the stone; to provide the necessary ions such as H, B, Be, Fe, Ti and Cr, in the case where inward diffusion is desired; to clean and heal fractures reaching the surface of the stone; and to make the surface of the stone easier to look through.
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treatment run and places them in a crucible. Weighing the exact amounts of chemicals used is crucial as it ensures that the mix can be accurately repeated.
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The stones are placed into ceramic crucibles containing a mixture of chemicals such as alumina powder, borax and boric acid, and the mixture is compacted in
chemical environment that will draw the excess iron out
rid of its brownish colour, leaving pure red. Similarly, removing iron from dark- blue sapphires may lighten the stone’s colour. In this case small experimental crucibles are being used instead of larger production-run heat- treatment crucibles.
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The crucible is topped up with stones and chemicals prior to the crucible lid being sealed in place.
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A ceramic sealant is used to fix the lids onto the crucibles. The stones sealed in each crucible are therefore subjected to certain closed environmental conditions dependant on the chemicals added, and are more or less independent from the internal environment of the furnace itself.
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A common and highly effective means of heat treating gemstones is using the traditional Thai coke furnace. It is a essentially an insulated, flued, chamber into which coke can be placed. It is driven by an adjustable electric air blower. This model and size of furnace
about 6 hours with its highest temperatures being maintained over a period of approximately 4
maximum temperature of about 1500-1600 degrees centigrade.
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With the lid of the furnace removed, the barrel, or combustion chamber can be filled with combustible
charcoal is added, as shown here, as this burns more easily and, in turn, will ignite the coke which is added
insulating fire-wool lining the inside of the barrel, and the drilled steel plate forming the base of the furnace combustion chamber are visible in this photograph.
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The lumps of coke are broken up into pieces of an even size and weighed into the furnace. Using the same weight of coke each time the furnace is run ensures that the temperature - time conditions are repeatable. Experiments showed that the actual size of the pieces of coke did not have a great effect on the run conditions. Only insofar as if they were too big then the pieces did not completely burn away to ash by the end of the run.
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The charcoal is lit using some petrol or kerosene and then the furnace is completely filled with coke. This is best way to ensure that the coke starts burning consistently and evenly.
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The small crucibles are placed within a larger, thick, black graphite crucible, which insulates the more delicate ceramic crucibles against thermal shock from sudden temperature changes as the furnace heats and cools. This method is called the “nested crucibles” technique.
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A view of the top of the combustion chamber loaded with coke and the gemstone crucible just prior to the lid being placed on the furnace.
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The ceramic sheathed thermocouple (above) measures the internal temperature of the
furnace air blower off and on thereby modulating the furnace temperature.
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A view of the top of the combustion chamber loaded with coke and the gemstone crucible. The lid is being placed
time, the thermocouple is slid in place between the lid and body of the furnace.
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During the heat treatment run, the crucible rides down towards the base of the furnace
crucible can be seen sitting on top of a thin bed of ash on the bottom of the furnace.
200 400 600 800 1000 1200 1400 1600 0:01 1:00 2:00 3:00 4:00 5:00 6:00 Time (hours) Temperature (deg C)
Short Boy (26 litre) Chimney Aperture modulated
Two different temperature-time graphs for the Short Boy furnace. The furnace can be made to run at a lower temperature for a longer time (red profile) by narrowing the size of the chimney
aperture during the run in this way, but it is easier to modify the speed of the air blower.
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200 400 600 800 1000 1200 1400 1600 1800 0:01 0:30 1:00 1:30 2:00 2:30 3:00 Time (hours: minutes) Temperature (degrees C)
Short Boy (26 litre) Baby Boy (3.25 litre)
The temperature-time curves are shown here for the 26 litre and 3.25 litre furnaces. Within certain constraints, the length of the hold time at the desired maximum temperature is dependent on the size of the furnace combustion chamber. Different sized furnaces are built for different purposes.
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200 400 600 800 1000 1200 1400 1600 1800 0:01 1:00 2:00 3:00 Time (hours: minutes)
Temperature (degrees C)
Short Boy 26 litre Tall Boy 52 litre
There was no gain in total run time by increasing the furnace combustion chamber capacity from 26 to 52 litres. As a result the Tall Boy furnace was only ever run twice.
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200 400 600 800 1000 1200 1400 1600 0:01 0:30 1:00 1:30 2:00 Time (hours)
Temperature (degrees C)
By using the thermocouple to turn the air blower off and on, the maximum temperature of the furnace can be held at any desired temperature. In this example a temperature of 1500 degrees was chosen.
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Hold Temperature 1500 degrees C
1480 1485 1490 1495 1500 1505 1510 1515 1:00:00 1:02:00 1:04:00 1:06:00
Time (hrs:min:sec)
Temperature (degrees C)
A detailed look at the flat top of the graph in the previous figure shows the saw tooth effect as the thermocouple turns the air blower off when the temperature reaches 1510 degrees and back on again when it drops to 1490 degrees centigrade.
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Here a crucible is opened revealing the effects of the heat treatment. In this case the stones appear lighter and brighter. Evidence of a reddish-brown stain on the surrounding hardened borax and alumina mixture suggests that this treatment may have had the desired effect by drawing trace amounts of iron out of these African rubies.
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Crucibles capable of holding about 600 grams of gemstones are used for “production-run” heat treatments. Here a sealed 600 g crucible of Tanzanian rubies treated with borax in the Coke Furnace has been opened for inspection.
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The cutting factory on the second floor of the Gemsellers building in Bangkok,
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Hand operated grinding wheel used for pre- forming the gemstones prior to the gutting and polishing process.
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Pre-form hand grinders are used to make pre-form
with Thai wax onto a dopstick (shown) and the pavilion (the stone’s lower half) is ground. The stone is then transferred onto another dopstick and then the crown (its upper half) is ground. The final dimensions of the stone are determined at this stage and can be carefully controlled using Vernier gauges, seen here lying on the small table.
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Pre-forming rough stones on the hand
is known as a “chut tang” in Thai.
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Both grinding of the facet and its final polishing are done on the same wheel. The stone is ground on the coarse outer portion then pushed to the fine inner portion of the wheel for its final polish. The entire process takes only a few seconds per facet. Around the wheel can be seen various cutting and polishing diamond pastes. The piece of paper wrapped around the lamp shields the cutter’s eyes from the direct light of the bulb being used to illuminate the job.
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The stone and dopstick are held in a “jam peg” (called a “muuy chap” in Thai) by the pressure of a tightened wing-nut (not visible in this photograph).
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After each facet is completed the dopstick is rotated to the next facet position, the amount of rotation being estimated by
the angles are not calibrated by machine, such stones are known as “hand cut” or “native cut” stones.
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The “Turbo Ten” production furnace operating in Chanthaburi Thailand. This model is now the preferred one used for the heat treatment of Australian blue sapphire. At 15 litres, it is midway in size between Baby Boy (3.25 litres) and Short Boy (26 litres) and seems to provide the most efficient use of the coke. Several of these units have been sold commercially in Australia and Thailand.
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The Turbo Ten is shown here operating. On top of the box in the backround, the temperature controller can be seen. The Turbo Ten furnace has a combustion chamber height of 280 mm and a diameter of 254 mm (Ten inches). The diameter of the chimney is 115 mm. These measurements exclude the insulation. As this furnace was designed to run at a maximum of only 1425 degrees, it was constructed
plate.
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