This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The block is now ready for firing. A furnace of fire-bricks is built round it, 2 chimneys being placed on the runner, and the vent communicating with the outer air, and round this furnace a second is built, in which a coke fire is lighted. The fire should be moderate at first, gradually increasing until the mass is baked throughout, so that it is completely red-hot to the very centre. After baking for 6 hours, the block is sufficiently heated to cause the wax to melt; this then escapes through the drain, which is in connection with an iron tube passing through the 2 furnaces, and communicating with a vat into which the wax flows. When the wax has ceased to flow, the opening from the drain must be carefully closed, in order to prevent any air from reaching the interior, which would be injurious to the process.
After 3G hours' firing, puffs of blue smoke are seen issuing from the chimneys. This shows that the heat is sufficiently intense to cause the evaporation of any wax that may have remained in the block. After GO or 70 hours the smoke changes from blue to a reddish hue; this shows that the wax is completely destroyed. The smoke is succeeded by a slight watery vapour, and the fire is increased until all moisture has disappeared. This is ascertained by placing a cold steel plate over the orifice, upon which the slightest vapour shows itself in the form of a veil or dewlike drops. If at this moment it were possible to look into the centre of the block, it would be found to be of a deep red. When all symptoms of moisture have disappeared, the fire is covered up, no further fuel is added, and the fire goes out gradually.
The external furnace is pulled down as soon as the bricks have cooled sufficiently to enable the workmen to do so without burning themselves; and in order to hasten the cooling of the block some of the bricks forming the cover of the interior furnace are also removed. Later this is also demolished, and the moulding block is allowed to cool. In a word, it is necessary to proceed gradually for the purpose of cooling as well as for that of firing, sudden changes of temperature being fatal, and the success of the operation depending in great part on the regularity of the process.
The firing being now finished, the block has the same appearance as before, only in removing the chimneys the runner and the vent are found to be replaced by holes or channels, while another hole will be found at the base in the place of the wax drain. The wax in melting has formed these channels, and has left a hollow space throughout the block between the core and the mould. Reference has been made above to the use of iron pins pressed into the wax bust. As long as the core, the wax, and the mould had not been submitted to the action of the fire they formed a solid mass, but with the melting of the wax the core has become isolated, and, as it is formed of exceedingly friable earth, the least motion might throw it down and break it; this inconvenience is avoided by the employment of the pins above referred to, which, penetrating through the wax, on the one hand into the core and on the other into the mould, render the core immovable even after the disappearance of the wax.