removing glue from slate tiles

Anyone know how to remove white ceramic glue from slate tiles? Got some off but it tends to scratch using a razor.
Any info would be appreciated,
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applied chisel-edged putty knife to remove it once it softens. There are also solvents for removing tile mastic, but some tend to be volatile, poisonous, or flammable, and may leave residual stains in slate, which is relatively soft and porous.
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