The tar was generated through microwave gasification of biomass. Although all generated tars quickly dissolved in acetone, this portion of the tar which was stuck in the reactor doesn't dissolve in acetone or propanol. This stuck tar was heated at a very high temperature and thus solidified and attached itself with the glass surface.
For such organic residues insoluble in organic solvents, the only solution to remove them is aggressive oxidation. You can try a mixture of potassium dichromate and sulfuric acid or a mixture of nitric acid and hydrochloric acid (aqua regia).
If the material is quartz, it is best to heat it to 900 or 1000 ºC in a muffle. At that temperature, all that residue will be removed.
As Mr Marquez suggests, but you might use and oxygen enriched atmosphere!
Thank you all for your answers. I didn't expect such a fast response. Appreciate the help.
There is a liquid called anisole - methoxy benzene. It is a better solvent than some you have tried. If you wish to know what the stuff is, then using a solvent is desirable because you can then do specific analysis of tar compounds. But combustion may be the final solution.
You may try different organic solvents until you find the right type.
Tar can be removed from Q.R by using Methanol after cooling down into room temperature.
To clean quartz or fused silica from graphite-like residue: hot conc. HNO3 or H2SO4; conc. H2SO4 + K2Cr2O7 (chromic acid*); conc. H2SO4 + conc. HNO3 (2:1 vol / vol) + KClO3.
(*) Note that 1 mol of potassium dichromate in presence of at least 1:2 mol/mol of sulfuric acid can be taken as equivalent to 2 mol of chromic acid referred to as CrO3: K2Cr2O7 + 2H2SO4 → 2CrO3 + 2KHSO4 + H2O. Indeed, no CrO3 is actually added, but rather half its molar amount as K2Cr2O7.
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