Cracking mortar

[GMK69]

Hi all I am at the 2nd course stage of my oven ,I have covered the outside of the bricks with a layer of mortar twice now but each time the layers crack and fall away. The first time I used a mix of 50\50 fire clay and sand which was then covered with tarpaulin ( was in Feb \ march ) second was with a pre mix of heat proof mortar which again has cracked. I don't want to progress any further until I get this sorted.
Any ideas and help would be greatly appreciated.

[Tscarborough]

Re: Cracking mortar

Fireclay and sand is not mortar, it is mud. If your premixed refractory mortar came wet and ready to use, it has a design thickness of 1/8" or less, and is not suitable for use as a plaster (or for building a dome oven, period).

For plastering the outside of the dome, normal pre-mixed mortar is fine, save the refractory mortar for laying the bricks.