Clay Slip Mortar for Batch Rocket

I have seen a lot about using clay slip for the mortar in a rocket stove and the brick bell.  What I don't understand is weather its supposed to bake hard from the heat of the stove or if its always gonna be a soft mixture that can be broken apart, remixed with water and reused.  If it never gets hard then the core witch is protected from shock, wrapped in ceramic wool and wire should be okay, but the bell is subject to abuse from the people in the room and movement of wood.  Right? I read that clay is fired at 1000F to 1400F degrees so the clay slip of the rocket core could get baked hard.  But the temperatures of the bell won't ever reach that high, so another mortar can be used right? I haven't found any information on the heat tolerance of lime mortar but i know it was used in fire places and brick ovens for hundreds of years.  What temperatures can I expect inside the batch rocket bell? thanks Lesleigh

Clay slip is to seal up the cracks and help to level the bricks, that's all. Its not a glue - it only seems this way when applied wet but once dry, its kinda hard and crumbly and will be easy to separate the bricks and scrape it off. Its not something that can be reused like cob. So little of it is needed though that its not a big deal.
Clay slip unlike refractory mortar, allows the bricks to shift around from expansion/contraction so expect cracks to form on the joints. This is normal.

Beyond the core, especially on the surface layer, cob with some fibre will help to provide the tensile strength to help hold things together. Easy to water down and reuse over and over again or repair.

I've never put a thermometer in my manifold area but I would say mine runs around 400F on average. Final vertical exhaust runs about 180F average. I measure this with a 'Dragon Breath' monitor (candy thermometer inserted into the stack). If you really want to know the nitty gritty of how clean your exhaust is, you could spend a lot of money and buy yourself a Testo gas analyzer.
Thankfully, Peter has already done significant testing on his batch rockets with his own device so I'm happy to take his word for it!

Ok So I contacted Lime Works in Telford PA and they told me that NHL 5 mixed with sand will make a mortar good up to 650 degrees celsius (1200 F).  So this should be good for a permanent bell mortar.  Natural Hydraulic Lime grade 5  

Thanks for all the input Gerry