The physical properties of oxalic acid may be of interest in this connection. The stuff one buys is usually oxalic acid dihydrate, which is a crystal which has two water molecules attached to each oxalic acid molecule. The CRC Handbook of Chemistry and Physics gives the following properties for oxalic acid dihydrate:

On heating:

1) The water of hydration leaves at 101.5° C (214.7° F) The water boils off leaving anhydrous oxalic acid crystals.
2) At 157° C (314.6° F) the oxalic acid starts to sublime (goes directly from solid to gas)
3) At 189° C (372.2° F) the oxalic acid which has not yet sublimed decomposes to formic acid and carbon monoxide.

Best regards,
Donald