Post 63 Ceramic dovetail

We all know about timber dovetails, those traditional little reverse wedge shapes used to connect pieces of timber in a really strong joint. Dovetails in drawers continue to be the hallmark of a well built and valueable piece of furniture.

But what about stone or ceramic. I got thinking about this recently when a pot came out of a bisque firing with a crack in the side wall. This is only the second time this has happened and it is a bit annoying. After having invested the time and energy to get it here it would be disappointing to throw it away, but to continue to go to a glaze firing would amost certainly result in the crack opening up and rendering the pot almost un-usable. I've looked for but haven't found an approach on the net about how to repair it for a reasonable glaze firing result. If anyone has an answer I'd love to hear it.

I think I know how the crack happened and how to avoid it in future; a question of differential drying stresses.

 In this photo the crack is at about 7 oclock, just left of centre on the rim. Here is a closeup. It would be a nice pot and I was making it for a japanese box forest.

So how to fix it. There are well established practicesboth new and old for repairing broken ceramics, but none of those methods will survive the changes and temperature of firing to maturity. What ever you use has to contract 6 or 7% with the body of the pot, retain the pressure on the crack faces and not burn up at 1230C.

The ancients used metal  'clamps' and dowels to join masonry in the days before reliable cement / mortar technology. The Greeks used pieces of iron set in holes in the masonry around which molten lead was poured to secure the joint, as above.

This one is a double dovetail or bowtie from Egypt at the ancient temple of Dendera

This picture is of the Temple of Apollo at Delphi in Greece and on the foundation blocks you can see the interconnecting iron clamps - installed like staples to keep the blocks together. This was an early technology to earthquake proof a building.

 And for the students of history can anyone tell me what this picture is all about. Well in the chaos of the post Roman world when iron production went into disarray you had to get it wherever you could. So the ancient world's ruins became both stone quarry and iron mine, where it was sometimes easier (cheaper?)  to chisel through the stones in a wall and extract the iron clamp than source freshly produced material.

I do remember too, though could not find a picture, of the stonework on Fort Denison in Sydney Harbour. As I recall stone bowtie joiners were used there to tie together the monumental stonework on the wall of the big gun emplacements. Perhaps this was commonly used in 18th and early 19th C defensive military stonework. If anyone has got a picture of this please send it to me and I'll post it.

 Well all very interesting indeed and mostly well before the time of bonsai so not really relevant. I agree of course.  
But with all that perhaps there is some precedent for me to fix my pot with a bowtie double dovetail clamp made of the same material as the pot.

So on the inside of the pot I've placed a bowtie that is about 2/3s of the wall depth and also drilled a small hole at the limit of the crack to stop propagation. Bisqued clay is nicely workable. Before firing I'll sprinkle a little galze in the base of the bowtie recess and around the join to secure it in position, then glaze right over the top both inside and outside. The crack may still be apparent, but after reaching maturity it will be totally stable and with luck the wall will now not gap open. Time will tell.

That's as far as I've got on this one and will now have to wait for the next firing opportunity. Speaking of production disarray I'm doing a few little renovations in my workshop at the moment and both pot making and firing is in total disarray!

This will likely be my last post for 2012. Season's greetings to one and all.