“…Ironbearing spinel crystals are chemically durable and are therefore less of a concern for the quality of the final vitrified product, when present at levels of up to ∼2 wt% [53] . However, these spinels can agglomerate and, due to their higher densities, settle to the bottom of the melter, which is intolerable in the JHCM melters as they are susceptible to clogging [54] , as opposed to induction melters, such as those used in the UK, where a modest level of solid settling is considered acceptable [55] . The glass product discharged from the DM100 melter test contained only trace amounts of crystal precipitation upon melting and cooling, and 0.65 vol% spinel crystals con- taining Fe, Cr, Ni and Mn oxides were measured after heat treatment at 950 °C for 72 h [47] which is broadly consistent with being within the acceptable range of product quality ( < 1 vol% of crystalline phases at 950 °C) [ 56 , 57 ].…”