Abundant Water

It is with a sense of relief that I sit here and write this entry. I was preparing myself for the worst. The news had come via email while I was away during my recent trip to N. America. The message was very brief ‘the latest pots have cracked’ was all it said. By itself that would not normally have caused such alarm.

So why was I alarmed. There were a few reasons. The latest batch of filters had incorporated a few changed that were likely to push the envelope. I had also spoken with one potting professional who provided some new information on our firing process.

The changes were:

1. To increase the firing temperature of the filters. This was because we discovered that the core of the pots was still black after firing. The black was carbon and indicated that the pots were not getting hot enough for long enough for the heat to reach the core.
2. To increase the size of the pots from the standard trial size of 600 ml to a three litre filter. This was in response to the feedback we received from the Ban Thalane visit and was the size that seemed most practical to their situation.

So we were firing bigger pots at a hotter temperature which increases our risk of cracking. Added to that I had also spoken with Mr. Burt Cohen from Potters Without Borders (a remarkable organisation doing great work to support the diffusion of clay pot water filter technology around the World). Burt has worked with this firing process previously and expressed some reservations over our ability to control either the temperature or the amount of oxygen during firing. This is because this type of firing, without a kiln, lacks any of the usual control mechanisms usually present in a kiln. Burt also suspected that the added fuel, due to the coffee inside the filters, would actually make the process more unstable. It all seemed to make sense so, although we had already fired filters successfully, I was a little nervous.

So that was the source of my anxiety. I owe today’s sense of relief to what I saw last night when I took a trip out to Noukham’s to see the the damage for myself. I was expecting carngage, perhaps even exploded pots or something equally dramatic. In reality, the largest filters were cracked in a few places and the smaller ones had some cracking but much less. It looks like this can be remedied with a few changes to our production process.

Here are the changes for the next batch of filters for the next batch of filters:

1. On the advice of Patricia Cheesman we are going to lengthen the firing cycle without making it any hotter. This is achieved through adding more straw to the insulation layer of the fire. This is where Nou Kham’s skill will come into play.
2. The filters cracked along joins. So we are going to use a more homogeneous construction method. This will remove the weak points from the filter.
3. Filters will be stacked differently for the firing. They will be laid out on a metal rack and there will only be one layer. We have observed that the filters are cooler where they have either been in contact with the ground or with another filter. I have been told this is due to path the flame takes around the filter.
4. The last change is in the shape of the filters. Noukham has fired pots that are at least thirty litres in size with here usual technique. The only difference is that we have coffee grounds in ours. The presence of coffee grounds seems to increase the amount of distortion that the filter undergoes during firing. We aim to make a filter with simpler lines and a more uniform thickness to reduce the competing stresses in the filter during the expansion and contraction phase of firing.

Fingers crossed, wish us luck and I’ll be back with an update next week