Mortaring firebrick

What is the correct way to mortar firebrick into a cast iron stove (A Waterford Stanley Ashing) The old bricks were set in a thin layer of what I assume is refractory cement? I had to remove them to replace a cracked inner back plate. Can refractory be mixed thin and used as mortar, instead of the thick consistancy that the directions request you to make it (Rutland). Or do you use something else to mortar in the brick and fill in voids in between them?

I may have said that incorrectly. They were not set into it, they sit on the bottom and sides and they the joints are mortared. If that helps at all. A very fine grained refractory was used in the joints in between bricks and cast. After years the refractory behind the bricks was no longer attached, it was just wedged in.

It may be very similar to that product.

This stove is a bit strange I guess. Not only was the firebrick mortared in, but the baffle, rockwool insulation, and back plate were all mortared into place.

I did get the stove used, to replace an old morso 2b. But I went over the parts diagram and it shows the fire brick installed how they were pulled out. The way they are installed leaves large 1"gaps. I also saw factory chalk marks on the castings so I dont think it's been apart. Unfortuentky the new inner back is also slightly smaller than the old so the bolt holes dont line up as well. (They arent drilled or tapped yet).

At this point I'm just trying to get it to work anyway possible.

It's a Waterford model Ashling non boiler yr 1999. I have found there were 2 Ashlings a wood/coal version in Europe and a wood only version in the US with no shaker grate. It's an epa approved stove as well. A lot of things were mortared. Namely the secondary air ducting. Insulation around it, and the entire firebox. I've called Lehmans for clarification but Waterford has more or less left the US market and information is scarce at best.