Fibers, Burn Out

Burn out fibers are added to both refractory castable and non-refractory fireclay mortar when casting a pizza oven dome to reduce the chance of steam cracking the dome during the initial, curing firings. The idea is that very fine, 19mm or so, 1/2″ long polypropylene fibers are added to the dome casting material at a ratio of roughly 1% to the dry mix before adding water. Because the melting point of the poly fibers is very low, about 320ºF, these fibers melt, leaving a network of very fine air gaps that allow any moisture in the casting material that is converted to steam to escape through the gaps without causing cracks that can structurally damage the dome.