Thanks for the help. I think I'll do what you said and leave it be for a day and cast the riser tomorrow. I have a fan on it and a dehumidifier running to help out a little. The more I think about it, the plywood mold is pretty well wedged into this core at this point and it will be supporting the riser, almost like a concrete anchor.
I am using a blend for the core. I'm sure I could try harder to find a recipe but just using what I have on hand and winging it. I started at the feed area and added more of the refractory cement but then teased it down towards the back. My reasoning is/was that I wanted the area around the feed to be super tough since that area gets so much abuse and not so worried about the rest. At the front end and down past the trip wire groove in the mold the mix followed this:
38.7% fire clay (by volume)
38.7% vermiculite
19.3% water
1.6% fire cement (dry)
1.6% refractory mortar
0.2% sodium silicate
I excavated the old core out down to the bottom of the burn tunnel and then added another 1"-2" on top of it so there should be around 5"-6" of insulation under the burn tunnel. Perhaps excessive. This bottom layer was 2-3 times higher in percentage of the refractory mortar, and maybe a bit higher in vermiculite too because I couldn't find a good container to mix in. Then I found the right container (I like to mix up small batches, about 4 gallons by volume at a time) and started to use the above mix. As I got past the feed and into the burn tunnel I started leaving out the refractory mortar- gonna save that for another project. I don't know how much of a difference these additions will make to the diy mix- not sure if I was adding a lot or a little.
I got the rest of the demo of the old stove out in about 2 hours- that was the easy part. Got the core cast today, laid the duct work in, fabricated the 8" duct to the old 6" cleanout ports, cut the hole in the chimney for the 8" exhaust and started filling the bench back up. I'm hoping by the end of the day tomorrow I will have all the duct work finished, fabricate the heat exchanger, plumb the water lines, cast the heat riser, and finish cobbing out the plenum, or plenums. I'd like to get a lot more mass back in the bench too but we'll see how far we get. This is where I left off.
There will definitely be some ongoing bits and pieces to work on- it will be a while yet before it's even cool enough outside to fire it up. so............
I'm really liking these 8" pipes. I didn't realize until I started working with it how much bigger they are. Also, the first time around used that register for the plenum and cut it back a bit- cobbing out the rough shape of it today I can already see that this is going to be so much better.
I set the lid from the grease drum over the hole where the riser will go and traced a small groove around it with a knife. Then I cut the wet casting material away to that shape on the two back corners. At first I was cursing myself for trying to cast this thing in place but once I started working on it, it went very well. I took to scraps of 2 x 6 and screwed them together to make an L and this worked great for packing the edges, forming corners, and making straight (ish) lines.
"Knowledge is power. Arm yourself."