Cut cucumbers in quarters, lengthwise. Dissolve salt in boiling water; pour over cucumbers in crock; weight down with plate; let stand 1 week.
Drain; add 1 gal. fresh boiling water; let stand 24 hours; drain. Add alum to 1 gal. fresh boiling water; pour over pickles; let stand 24 hours; drain.
To make syrup, combine vinegar, sugar and allspice; heat to