In the determination of provenience of obsidian artifacts, precise and accurate measurements of composition patterns of the geological sources are necessary for definitive and cost-effective assignments. Intercomparison of data from different laboratories is often difficult. Suggestions for maximizing the usefulness of data already in the literature are made, contributions to a useful data bank of source composition patterns are recorded, and provenience determinations of 30 artifacts excavated in Quirigua, Guatemala, are presented to exemplify the technique. -- AATA