Inner Hearth Refractory Build – Chapter 1-5 Released

Prototype Gasifier Build

Today I am releasing the first 5 chapters of the Refractory Build Videos.  You can see how I Cast my first Refractory Inner Hearth.  I learn both what to do and not to next time, but Overall I am happy with the results and will be using the first test pour article in my initial prototype build.

The two main lessons learned are:

1.  Make sure to add enough water to the refractory mix, so that it is pourable and not clay like.

2.  The refractory mix will stick to the metal form, even with oil applied before pouring.

Follow my progress below:





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3 comments… add one
  • John, I hate to break it to you, but the Portland Cement will break-down at 850F or so ~ All castable refractories use alumina-silicate cement, which initially forms hydrolic bonds, that get converted to ceramic bonds with temperatures above 1200F. The mixture ratio of cement-to-water you used looks quite damp compared with the mixtures I have used (as little as 3% moisture by weight). The secret is very thorough mixing in a beating mixer and sufficiently powerful vibration of the right frequency to turn it into a slowly-flowing fluid. Without the vibrating you’re dead in the water, so to speak. The more water you use, the weaker the refractory. I recommend using a premixed industrial refractory mixture…..they have lots of experience behind their instructions! For mold-release, I have successfully used several coats of floor-wax, which doesn’t rub-off like oil. …..and bolt or clamp the mold pieces together with flanges that allow for easy rolling off of the metal without stressing the ceramic.
    You are right about the outside piece of metal not being a problem ~ it will expand much more than the refractory and come loose with heat ~ this same quality will probably expand the inside piece too much and crack the refractory. Live and learn!

    • Jfedock

      I wonder what component of the cement burns off at 850? I figured the cement would weaken with increased temperature, but didn’t plan on the binder burning out. Either way I agree its probably not the best long term solution. As you will see in the next series of videos, the mixture was still TOO thick to pour in to my mold. I like your idea of using a mixing beater ! I did use the vibration table (tire) and it seemed to work well on the large flat forms, but not so well on my small constricted ones (the hearth).

      I am going to use my “sheet metal” hearth for the initial prototype testing. I will re-cast another hearth if required in the future. This test will be of interest only using a sheet metal hearth, they will be able to better understand how long carbon steel will last at high temperatures. IF it only lasts a short time before oxidizing then we will see how the concrete holds up. 😛

    • Hey John, me again with a thought on your vibrating table. The way your vibrating motor is mounted imparts a circular motion with horizontal axis. This makes your casting mix bounce with gravity, not a settling movement. The vibrator table I made consists of a square table mounted to 4 thin vertical rods welded to a base plate. The vibrator (a concrete stinger) is mounted vertically, firmly clamped to one of the legs. This imparts a circular vibration oriented horizontally, which tends to jiggle particles into their tightest configuration while allowing bubbles of air to escape (important). A concrete vibrator will allow the use of a rheostat for speed control.

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