Paste should cool before use. Diluting and Storing Paste Different consistencies of paste are required, depending upon the particular mending task at hand. A consistency similar to heavy cream is adequate for most mending. Pastes should be diluted with distilled water to achieve the consistency required. It will keep for only a week or less.
Some conservators recommend adding a preservative.
The preservatives used, however, are toxic. It is preferable to make paste in small quantities when it is needed rather than add a preservative and store it for long periods. If paste discolors, grows mold, or develops a sour smell, discard it immediately. Discard it if dark flecks appear in the paste since they may indicate mold or bacterial growth. Methyl Cellulose Starch pastes require time to make and thus are not practical if they are to be used only occasionally.