Sometimes the cloth gets so dirty you can see it. Or it even re-greases your lenses! All the dirt, dust and fat rubs off on the cloth. This is exactly what the cloth is made for. After a while it is so dirty it will not clean your glasses properly anymore. What to do about it?
Simply shake out the dust and dirt.
Wash by hand
Use cold water. Add a few drops of mild soap that is bleach free and which does not contain fabric softener. Let the cloth soak in the soap for a few minutes and then rinse it clean in cold water.
Wash in a washing machine
Use bleach free detergent, and it should be free from fabric softener. Let the cloth air dry when finished.