The　optical　erasure　dynamics　in　a　batch-thermal　fixing　scheme　of　holographic　storage　in　photorefractive　crystals　is　investigated　theoretically　and　experimentally.　The　inter-batch　optical　erasure　time　constant　T-F　is　introduced　to　specify　the　optical　erasure　to　compensated　gratings,　and　measured　in　a　sophisticated　experiment.　The　experimental　result　shows　that　T-F　is　much　longer　than　the　intra-batch　optical　erasure　time　constant　T-E.　The　difference　between　T-F　and　T-E　is　fundamental　for　enhancing　nonvolatile　storage　density.