I found a refractory repair material that impressed me.

I found some cracks in the firebox of the fireplace at my rental house. I needed to reseal it for safety, to restore the function of the gas logs.

I went to the lumberyard for refractory cement, and found this:

This fits in a standard caulking gun, and dispenses refractory materials.

It only needs a wire brush for preparation, then it dispenses like bathtub sealant:

Since it is water based, it can be smoothed out with a finger; which forces the compound into the opening, and increases the contact area with the original brick.

This seal was harder to reach:

It filled well, but used a lot more compound to close. There was enough in this tube to do the entire repair. This repair took about a Fourth of the time I expected, and looks like it will be a better fix. The bonding is a lot better than straight refractory, and will resist almost 1400 degrees F; so the gas logs are overmatched!

This is my new favorite use for a caulking gun!