When a crystal is delivered it carries with it a few monolayers of adsorbates from ambient, typically water and some hydrocarbons. The carbonaceous content of these adsorbates will be annealed into the near surface portion of the bulk of the crystal if the crystal is immediately annealed to very high temperatures prior to any sputtering. After such an initial cycle, it will become virtually impossible to remove it through sputtering and annealing as it spreads further through the bulk with every anneal cycle. Silver, with a small, but non-negligible bulk solubilty for carbon, is a notorious case where this has a tendency to happen, and it can in cases be wiser to repolish a crystal to fully remove the carbon-poisoned layer of the crystal instead of persisting in removing the impurity through extensive sputtering/annealing.