I love the 5 minute riser. I have to thank whoever thought this up and they should be blessed for introducing us to something with both durability and ease of construction. Recently I've seen here and on donkey's board that folks are concerned with using the 5 minute riser in black oven applications because of the risk of the ceramic fibers breaking free into the fire path and being deposited into their food. Well I have a solution that builds on the 5MR concept that improves heat retention in the riser as well as sealing the riser to eliminate any chance of fibers escaping.
I did this with my riser and after 20 some odd firings, some of which were as hot as I could get the stove, the inner surface of my riser is still perfect. The addition is a coating of refractory material called Satanite that I purchased here: High temp tools: refractory products
. It is mixed with water to a mayonnaise consistency and spread on the ceramic fiber. It is better to do it in two or three thin layers than one thick layer and one pound will do two good coats in a 5 inch ID riser 24 inches long with some to spare for touchups. I mixed it in several small batches because I couldn't estimate the amount needed to do one layer and the stuff tends to be sensitive to working time. I found it best to mix only what I could spread in 5 minutes or so, which meant four batches per layer.
The Satanite dries to a hard plaster like material that then needs to dry for at least 24 hours but I gave it 48 just to be sure. Then you heat it up a little at a time to drive out the trapped water. I used a propane torch to do this in about 8 passes. Each time it began to steam I'd stop and let it cool down, then do it over again. The last time I heated it up for about 15 minutes and no more steam was seen escaping. After that I did some light fires in the rocket stove for the next 5 firings. After that I paid it no more attention other than to marvel at the heat retention and durability under intense heat.
I hope this helps anyone needing a better environment for a black oven or just a high heat retention riser.