The last dehydrating alcohol must be 100% to ensure no water will be carried into the following clearing agent. If the clearing xylene is contaminated with water, it will appear milky-white, and the slides will not clear.
Remedy:
Once the slides have been contaminated with a clearing agent, the only remedy is to bring them backward to water, dehydrate them, and clear them again in fresh solutions.
The Coplin jar's xylene on the left side of the image is contaminated with water. Note how cloudy it is compared to the one on the right, which is not contaminated.
