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 mounted on the base of the combustion chamber and blows air directly upwards into the combustion chamber. Note the flame visible at the top of the chimney. 2
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 3
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. 4
Mr Geoffrey Sobels and Mr. Kittisak Manepong in the ground floor 5 office of the Gemsellers building in Bangkok, Thailand.
Specifications for the furnaces designed by Mr. Geoffrey Sobels and Dr. Robert Coenraads.
Two of the furnaces designed by Dr. R. Coenraads and Mr. G. Sobels. These 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. 7
The “Tall Boy” furnace (left) has a 52 litre combustion chamber created by the addition of an extension sleeve to the Short Boy furnace. The 3.25 litre “Baby Boy” furnace is shown at right for comparison. 8
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. 9
The finished “Baby Boy” 3.25 litre furnace, being held by one of the 10 engineers, illustrates the light weight and transportability of this model.
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 of this wool, especially after it has been heated. 11
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 12 always be kept for later comparison with the final product.
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! 13
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 14 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.
Mr. Geoffrey Sobels mixes the gem stones and various chemicals for the heat treatment run and places them in a crucible. Weighing the exact amounts of 15 chemicals used is crucial as it ensures that the mix can be accurately repeated.
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 place. The aim is to create a chemical environment that will draw the excess iron out of the stone and thereby get 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. 16
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The crucible is topped up with stones and chemicals prior to the crucible lid being sealed in place. 18
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 19 the chemicals added, and are more or less independent from the internal environment of the furnace itself.
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 operates for a period of about 6 hours with its highest temperatures being maintained over a period of approximately 4 hours. It reaches a maximum temperature of about 1500-1600 degrees centigrade. 20
With the lid of the furnace removed, the barrel, or combustion chamber can be filled with combustible material. First a little charcoal is added, as shown here, as this burns more easily and, in turn, will ignite the coke which is added afterwards. The white 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. 21
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. 22
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. 23
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 24 “nested crucibles” technique.
A view of the top of the combustion chamber loaded with coke and the gemstone 25 crucible just prior to the lid being placed on the furnace.
The ceramic sheathed thermocouple (above) measures the internal temperature of the furnace. The controller unit (below) displays and records this data and can switch the 26 furnace air blower off and on thereby modulating the furnace temperature.
A view of the top of the combustion chamber loaded with coke and the gemstone crucible. The lid is being placed on the furnace. At this time, the thermocouple is slid in place between the lid and body of the furnace. 27
During the heat treatment run, the crucible rides down towards the base of the furnace on top of the burning coke as it reduces in volume. At the end of the six hour run the 28 crucible can be seen sitting on top of a thin bed of ash on the bottom of the furnace.
Short Boy Furnace 1600 1400 Temperature (deg C) 1200 1000 800 600 400 Short Boy (26 litre) 200 Chimney Aperture modulated 0 0:01 1:00 2:00 3:00 4:00 5:00 6:00 Time (hours) 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 opening. The temperature can be raised or lowered by closing or opening the chimney aperture during the run in this way, but it is easier to modify the speed of the air blower.
Unmodulated Furnace Runs 1800 1600 Temperature (degrees C) 1400 1200 1000 800 600 400 Short Boy (26 litre) Baby Boy (3.25 litre) 200 0 0:01 0:30 1:00 1:30 2:00 2:30 3:00 Time (hours: minutes) 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 30 size of the furnace combustion chamber. Different sized furnaces are built for different purposes.
Unmodulated Furnace Runs 1800 1600 Temperature (degrees C) 1400 1200 1000 800 600 400 Short Boy 26 litre 200 Tall Boy 52 litre 0 0:01 1:00 2:00 3:00 Time (hours: minutes) There was no gain in total run time by increasing the furnace combustion chamber capacity from 31 26 to 52 litres. As a result the Tall Boy furnace was only ever run twice.
Short Boy Furnace Production Run 1600 Temperature (degrees C) 1400 1200 1000 800 600 400 200 0 0:01 0:30 1:00 1:30 2:00 Time (hours) By using the thermocouple to turn the air blower off and on, the maximum temperature of the 32 furnace can be held at any desired temperature. In this example a temperature of 1500 degrees was chosen.
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