I can't say side blast makes for fewer sparks, but side blast works better for charcoal. A bottom blast will tend to blow the lit charcoal right out of the fire pot unless you have almost no blast which will leave the fire on the cool side for forging. Of course if you just run a pipe into the fire for the side blast the pipe will want to burn up.
For my set-up, I find 4-6" fire depth works about right. That gives me a hot spot a little bigger than my fist (roughly a 5" sphere).
If I was making a charcoal forge, I stick with castable refractory or fire brick (and I plan to as soon as I get some $$$). But before I finalized plans, I'd try different sizes, shapes, number and construction of blast pipes etc using ash or clay to decide what suited my purposes best.
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