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Re: Need advice on chimney and outer arch construction
Max,
You could form the clay pipe inside the PVC pipe and allow it to dry. Around 25mm should be a good thickness. It will shrink away from the pipe as it dries and you will be able to remove it easily. If you form over the pipe it will shrink as it dries and make removal difficult if not impossible and if allowed to dry completely will probably crack. Depending on the type of clay and the grog content, expect around 5% shrinkage from plastic state to dry and a further 5% after firing. However there is an alternative which does not involve making the pipe in two pieces. You can wrap and tape the PVC pipe in a layer of plastic, form your clay around the outside of the pipe and then roll the whole lot over a few times. This will increase the diameter of the clay pipe and you can then slide out the PVC one. It will be quite difficult to fire the clay pipe to 1000 C safely without cracking it, especially if you intend using a wood fired kiln. A clay pipe of this diam should be quite servicable but to avoid cracking it you should insulate it. Not good to have it cold on one side and hot the other. You'd probably have more success forming one using home brew over a sand mould. You could cast the whole vent and chimney in place at the mouth of the oven. It works I've done it. When done remove the sand.you just need to get the mix right so you can hand form it so it stands up vertically. Make it around 25 mm thick and add some stainless steel fibres and polypropylene fibres ( not absolutely necessary) if you can get them.
Dave
Max,
You could form the clay pipe inside the PVC pipe and allow it to dry. Around 25mm should be a good thickness. It will shrink away from the pipe as it dries and you will be able to remove it easily. If you form over the pipe it will shrink as it dries and make removal difficult if not impossible and if allowed to dry completely will probably crack. Depending on the type of clay and the grog content, expect around 5% shrinkage from plastic state to dry and a further 5% after firing. However there is an alternative which does not involve making the pipe in two pieces. You can wrap and tape the PVC pipe in a layer of plastic, form your clay around the outside of the pipe and then roll the whole lot over a few times. This will increase the diameter of the clay pipe and you can then slide out the PVC one. It will be quite difficult to fire the clay pipe to 1000 C safely without cracking it, especially if you intend using a wood fired kiln. A clay pipe of this diam should be quite servicable but to avoid cracking it you should insulate it. Not good to have it cold on one side and hot the other. You'd probably have more success forming one using home brew over a sand mould. You could cast the whole vent and chimney in place at the mouth of the oven. It works I've done it. When done remove the sand.you just need to get the mix right so you can hand form it so it stands up vertically. Make it around 25 mm thick and add some stainless steel fibres and polypropylene fibres ( not absolutely necessary) if you can get them.
Dave
. Lately I have become fond of disposable latex exam gloves. They are relatively cheap, if you can get them in the box of 100. I still use a scented variation of mineral oil, though . It is called "baby oil" 
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