The rate of hydration is not uniform throughout the world. Variations exist in temperature over time from site to site. Temperature effects are particularly difficult to evaluate. Variations also exist in sample chemical composition. Samples from different obsidian sources hydrate at different rates.
Moisture is another source of variability.
The amount of moisture present at a site can affect the hydration rate of an obsidian sample. It is really necessary to produce a calibration curve for each archaeological site or area being studied, and this is not always possible. Artifact reuse may lead to an erroneous date. For example, one person fashions a tool out of an obsidian nodule and uses it to skin a deer. Once that person finishes using the tool, it is discarded.