pindmiste gaasitühikute moodustumine valandi või valuploki pinnakihtides valuvormi halvast gaasiläbilaskvusest tingituna (vt pindmine gaasitühik), the formation of blisters - voids in the casting material, possessing a fairly large volume compared with common blow holes, and an elongated shape, which usually occurs just below the top surface of the casting (or ingot) and in poorly vented places (see: blister)