The present invention teaches an alginate band fastener device which may be inserted across the alginate band and tooth void of the gutta percha negative mold, then left in place while the mold is filled with gypsum, plaster or other hard material and vibrated, the alginate band fastener holding the band in exact position during the entire process. The fastener may have a cross bar, two or more barbs extending downward from the cross bar, and at least one detent aiding in clasping of the alginate band. In embodiments, there may be four barbs extending downward in pairs, each pair clasping one side of an alginate band, there may be two barbs both of which clasp a single portion of an alginate band, or there may be two barbs which generally sit across the alginate band on distal portions of it.