Vent Casting

[swripley]

I included this question in my recent posting to Brick Oven Photos, but wanted to be certain it didn't get lost so decided to post here as well.

I will be casting my vent out of KS-4 refractory castable. One of the issues I read about in postings regarding refractory castable is their lack of strength. All say not to worry as it will only be holding the chimney and not bearing much weight. I am also angling my brick cuts. All of the bits and pieces I have from cutting bricks got me to thinking. Could I break these up and use them as “aggregate” in my KS-4? I was thinking that it might add some strength as well as stretch the refractory as I’m worried that I have enough to fill the form.

Thanks,
Sharon

[wlively]

I cast mine from KS-4. Dang hard and dense stuff, so I don't see strength as an issue, especially if you throw in some stainless steel needles for reinforcement. It spec's at a crush strength of 2650 lb/sq in at 1000deg F and a rupture strength of 600 lb/sq in at the same temp. I am not so sure about mixing the brick bits in. The material make-up of the bricks is not too far off from KS-4, but it is a significant difference. KS-4 is 41.9% silica and 45% alumina. For the same reason as you mentioned, I put a complete brick in the center of my KS-4 top dome plug. I cut the brick sides sloped in case the bond between the two ever got questionable. To date, which has only been 3 high temp pizza firings, everything looks great.

I had enough in one 50lb bucket to cast my vent transition (approx 12" tall) and my dome plug with absolutely nothing left over. If you are only doing a vent you should have more than enough. It also is strong enough that you don't need to make the vent very thick. In some spots on my transition, the vent casting is only about 1/2 inch thick.

Keep in mind you need to cure the casting in an oven with a very specific temp schedule. You can cure in your house oven as you only need to reach a max of 500deg. I have a copy of the cure schedule. And also, do not use aluminum foil to line your form, it sticks/bonds to KS-4 and took me forever to get it off. Platisc wrap works very well.

[DrakeRemoray]

Could use use form oil to keep it from sticking to the form?

[wlively]

Possibly, but I did not want to take any chances of anything being absorbed into the casting. I switched to saran wrap on the dome plug and that worked great.