You can clean the dirt off with a little water on a cotton swab, but make sure to dry them off good with a soft cloth or tissue. Deffinetly segragate them from your other dice. You might want to consider putting a clearcoat on them. Some collectors do not want to clearcoat older dice as they consider it to damage or devalue them. I clearcoat all my crystalizing (nitrate) dice that I am going to keep in my collection. It not only considerably increases the life span but also males it safe to handle dice that can become extreamly fragile. Clear nail polish works and I have had good results with clear spray on laqueur. Use gloss for polish dice and flat for sand finish dice.