Although quite rare, in cases where this happens, it’s because the water being treated was already very clean. What’s happening is that some of the sulfate minerals in the Biotite Concentrate are naturally attracted to the glass. This is because really pure water has a pH that is slightly acidic, like 6.7-6.9. When the slight acidity comes in contact with the liquid minerals in the concentrate, the resulting salts are drawn to the glass. The stain is yellowish because the minerals are in a sulfate form and sulfate minerals are yellowish by color. (Sulfur itself is very bright yellow). However, the stain on the glass can be easily removed since it just wipes off. Another option is to use less Biotite Concentrate since you now know that your purification system is doing a really good job. Of course, if you want your drinking water to have a full complement of ionic minerals, you’ll have to decide how much concentrate to use. Again, a bit of experimentation in each situation can bring about a good compromise between pure water and a balance of ionic minerals. After all, Mother Nature always adds other things to water. Some examples: The Great Salt Lake in Utah has ionic minerals in the water, as do the oceans of the world. Mineral springs found in many places around the world also contain an abundance of ionic minerals, and so too, can they be found in high altitude waterfalls and other pristine aquatic environments. To a degree, it’s true to say that Biotite Concentrate is a convenient way to recreate the naturally occurring ions that are present in the waters of the world.