Longmont, CO new WFO build - casted over sand

[cnegrelli]

Just about at the end of forming the dome with Castable. Cast Max 28 was the product.

[cnegrelli]

removed form this morning. Casting looks pretty good.

david s I will have 3-4 days before I can do the gallery/flue area. Should I leave sand in until then, thru that time, or beyond? Currently keeping it wet and covered with plastic. Hard as a rock this morning.
Also didn't expect it to be lava black!

[david s]

You may have some voids to fill on the inside so keep the casting damp until you remove the sand. I sieve out the larger aggregate and make up some mix to a peanut butter consistency for doing this operation.

"Weak as in 5:1?
How about a Vericulite / Refractory mix for this application? I am fresh out of concrete mix."

Yes, but sieve out the aggregate from the castable so it's cement content is greater, then use 6:1 vermiculite, sieved castable. That should give you a material that will hold together but have some give.

[cnegrelli]

Thanks. What tool works best for filling voids on inner dome?

[david s]

I use an old butter knife, then rub the surface with finger tips.

[cnegrelli]

Back from business travel, I was able to work on the WFO this weekend. Because of the earlier mishap whereby the entry sand mold collapsed during the process of putting on the refractory, I'd been thinking about the impact for that. Partially removed sand from the dome entry area and ascertained that everything ended up fine to that point, with the exception that the right side of the gallery had shifted forward just under 1" compared to the left side. In other words, my "inner arch" is not square to the front area.

So I resolved to make my thermal break a bit wider and make up the "squareness" using varying thicknesses of the vermiculate and ceramic rope "joint" that will join the Entry to the Dome.

I then decided to build a mold separate from the dome (rather to cast-in-place up against the dome. The dome was completed 6 days ago.

Mold built that uses plywood to hold up the gallery sides for the first 6.5" and then uses Sand to slope up the arch and also create a funnel for the flue. Topped it off with a cardboard template that mimics the Chimney Anchor Plate I purchased from Forno Bravo. Stainless hardware embedded. Found a windshield fluid bottle which was just slightly larger than the anchor plate's opening (6" diameter) so it should fit comfortably with room for thermal expansion.

[cnegrelli]

post casting

[cnegrelli]

Mould I built. This allowed me to push castable down into the two sides of gallery, but then have ability to free-form build on top of the sand, to get the smooth transition from gallery/entry up to 6" ID chimney flue.

Forgot to use wet newspaper when we built the dome, seems to be fine though.
Went to use the wet newspaper yesterday, but we had problems with it laying flat and sticking, so we removed it and went directly over sand again.

[sriceky]

Love the HF cement mixer! I've used mine already for the hearth. I am actually building forms for my dome (I know the sand castle is quicker, but I want to build the forms) and should cast my oven over the long 4th of July weekend. Guess I should start a build thread.

[cnegrelli]

Pulled off the outer moulding (all but two pieces of board that help form the bottom portion of the inside arch of gallery. White stuff is styrofoam filler. Then sand on top. All looks pretty good overall.
This pic shows detail of the inner part of the gallery arch (how I used wood and sand).

[cnegrelli]

On this photo, along the top of the arch, it looks like a crack in this picture, but it's really just more like a void or where two different "layers" came together. If I recall right, I can mix up some more and do a little surface smear on there for appearances? That section will be visible even after the enclosure build.

[cnegrelli]

Is there a good reference for curing for castable on the site? I've read various things. Dome was cast a week ago today. I think I will dig out the sand and let it sit a week to dry out now? Then wrap in insulated blanket and start small curing fires?

[david s]

Proprietary castable refractory achieves full strength in 24 hrs so prolonged damp curing is unessessary. Driving out the water is another thing, so look at your product's recommendations for first firing schedule. It will be something like 25 C / hr up to 100 C hold at this temp for a couple of hours before ramping up to 200 C at 25/ hr and the same again up to 300 C and again to 400 C. The water elimination and firing schedule is also dependant on casting thickness. Also read the thread on curing under the "oven management, firing your oven" thread.


Here's a good link for firing schedule.
http://www.darleyfirebrick.com.au/castable-use.html

[cnegrelli]

OK - that's good news. I'll dig out the sand tomorrow and let them dry for another 48 hours and then start the curing fires slowly. The schedule for the Castmax 28 product I used has this schedule, which seems aggressive. I'll be more patient.

[david s]

If you see visible steam you're going at it too hard, back off.