raised up to 900–1100 1C (depending on REE concentration). The
melting time was 90 min. Melting at 900 1C allows addition of up to
2 wt% of REE, and at 1100 1C up to 8–5 wt% of REE can be added. The
homogeneous melt was poured into the heated up to 200 1C glassy
carbon crucible and cooled at room temperature. The prepared
samples were colorless glass plates with a thickness of 3 mm.