[Neil2]

All refractory mortars I know of will shrink somewhat. If you are spanning a joint of more than 1/4 inch or so expect cracks. If you want to span a larger joint - extend the mortar with a sharp sand (mortar sand or silica sand). For 1/2 inch span, go with a 1 to 1 ratio, 1 inch span try a 3 sand to 1 mortar mix ratio. If you have tapered joints, try having two mortars - "neat" for the tight part and "extended" for the wider part.