Hi Nyairo,

Soak the platinum crucible in hydroflouric (HF) acid for 24 hours.. that should dissolve any fused glass from the crucible.. 

Hope this helps.

To the Adel's answer I suggest to add some HCl to HF in order to better solubilize the salts and to heat to about 100°C until dryness, then rinse with water to take off the reaction product and repeat the attack more times mainly if the glass layer is thick.

You may check next reference (p. 98; in Portuguese), which addresses cleaning of platinum crucibles from molten glass ― without using HF: https://www.researchgate.net/publication/231157960_Cristalizacao_de_Biomateriais_Vitroceramicos_e_Mineralizacao_em_Meio_Fisiologico_Simulado

Thesis Cristalização de Biomateriais Vitrocerâmicos e Mineralização...

you can add Na2CO3 and melting with the remaining  glass at  800 C then in HCL 10 Min the crucible will be very clean

Thank you so much for your responses. I was able to clean the crucibles using HF. Many thanks for your responses.I would like to prepare fused glass as a matrix to host some nuclear fission products I am intending to study. I have platinum crucibles that I simultaneously use as crucibles and molds. I mix silicon oxide, sodium carbonate, calcium carbonate and aluminium oxide (glass forming matrix) with lithium metaborate in 5:1 flux to sample ratio. Then, I add 40mg of KBr to act as a releasing agent. I use an electric furnace and fuse at 900 degree Celsius for 2 hours. I obtain a nice glass melt however the glass sticks to the crucible and I get stuck. what should I do to attain a fine fused glass for XRF analysis of these fission products.

Pour the melt into a flat graphite crucible and then anneal at about 450-480°C to prevent glass breakage. You must heat the graphite crucible before pouring the glass and then slowly decrease the temperature. There are special cilindric graphite crucibles with a flat engraving that let you obtain a good flat glass disc. It is likely you have to polish a little the glass Surface to analyse. Best regards