My little J is going to get a little shorter. I'm going to cut the top of the tunnel off the original and fit a new core into the soft insulating material that is left in the metal box.
Here is a mold of a shortened 6" J.
![Image](http://i1320.photobucket.com/albums/u529/bobwieser/1427103242174_zpsabz40nzq.jpg)
![Image](http://i1320.photobucket.com/albums/u529/bobwieser/1427103266335_zpsuzvsjudl.jpg)
Those are 2 pics of the same side. It will have 2 halves.
The core will be 1 1/4" thick except the top of the tunnel will be 2" thick.
I have an angled trip wire, a cutout in the feed for a P-channel, and a pointed kick tail.
I have dense refractory rated to 3200 F. I want it to be strong so I can move it at times. Or replace it for experiments with castings from granite dust mixes I have been tinkering with.
I'm tempted to add perlite into the refractory mix for insulation and volume to cast both halves with 1 50# bag.
Any suggestions for how much perlite to add and keep the strength? I hope to cast before the end of this weekend.