Figure 1 shows a range of different colours associated with a wide range of temperatures to which the surface has been subject, with that range being a function of packing, fuel type, the direction and strength of any prevailing wind, as well as the duration of the firing. This temperature variation indicates a potential problem for the production of a strong, permeable, sintered ceramic filter, as it indicates lack of uniformity in density and permeability, with probable variation to post-sintered strength.

Those areas that are black and grey, [2]. [3], [4], [5], indicate that the surface temperature at those points can be regarded as not having exceeded 700°C, with carbon deposited on the surface by combustion remaining on those surfaces as a consequence. It can reasonably be assumed that beneath those regions combustion of organic materials contained within the pot’s cross section will also be incomplete, with corresponding consequences for both permeability of the structure as well as development of an approximately uniform density of the sintered structure. Permeability will be variable across the walls of the pot because of the probable non-removal of organic material, while density will also be lower there than in surrounding areas where higher temperatures will have produced a more open void fraction because of more extensive, hotter sintering and contact-point fusion of constituent particles.

The rim of the pot [1] shows an approximately even colour that is associated with both an oxidising flame and surface temperatures sufficiently higher than 700°C to ensure the combustion of carbon on at least the surface of the vessel. It is probable that the core of the vessel’s wall will still contain residual carbon with reduced permeability as a consequence. See Fig 3, [5].

Macro voids are seen on the surface of the rim. These will be a consequence of the making process and the combustion of organic material during the firing.

It may be assumed that the voids that can be seen on the rim’s surface, [4], [8], will exist with variation to their distribution though out the pot’s cross section and it was to reopen that void fraction that I suggested scraping the surfaces of the pot when dry. But regardless of the efficacy of that, if the carbon is not removed in the firing, [5], the pot will not be able to achieve uniform maximum permeability.

Macro voids are seen on the surface of the rim. These will be a consequence of the making process and the combustion of organic material during the firing.

It may be assumed that the voids that can be seen on the rim’s surface, [4], [8], will exist with variation to their distribution though out the pot’s cross section and it was to reopen that void fraction that I suggested scraping the surfaces of the pot when dry. But regardless of the efficacy of that, if the carbon is not removed in the firing, [5], the pot will not be able to achieve uniform maximum permeability.

Figure 3, [4], [6] shows the broken cross section of a pot broken in transit, with two layers of oxidised sintered clay in which carbon from organic material additions has been successfully removed. The differences in the thicknesses of the external and internal oxidised layers can be attributed to differing time periods over which the respective layers were exposed to flame >700°C, and in the case of the internal layer, because of its relative inaccessibility to flame path and direct heat.

When fired in a conventional kiln a ceramic object is exposed to increasing heat that is delivered via flame pathways, where combustion gases and flame will move though a pack, flowing around and over surfaces in much the same way that water moves in its passage over and around rocks. Water is said to follow the path of least resistance and this is similarly true with flame, so the packing of objects then becomes a determining factor in flame-path development and direction. A second factor to be considered is that the temperatures achieved in these low-temperature Lao firings are significantly lower than those temperatures, ≈1150-1200°C in a conventional kiln, at which heat ceases to be delivered via flame pathways and where the whole ceramic-pack mass assumes a much narrower temperature-range variation and energy is distributed through out the pack by radiation. This means that the following factors then achieve importance greater than they would in a conventional chamber: the placement of objects relative to each other and the consequent development of flame pathways, the volume of fuel available and its water content, prevailing wind speed and constancy, the total elapsed time of the firing and the relative humidity of the air available for combustion.

So, in summary:

• The pots need to be fired hotter and longer
• Their method of packing needs to be such that more of the internal surfaces of the forms are available to flame pathway development, this can be facilitated by stacking the forms on their rims, so that the interior of the forms are open and not sitting like upside-down cups.
• The firing will need more fuel and that might have to be added during the course of the firing, with it not being assumed that fuel heaped over the objects being fired at the onset will be adequate and not need replenishment (this might already be being done, I have no way of knowing.)
• The wall thickness of the filter structure might have to be reduced and made more uniform, so that uniform combustion of carbon is achieved

By Dr. Tony Flynn.