What is the stuff at the bottom of the cork? Dark red, almost inky, deposits, sometimes crystalline in texture? You find it at the end of the cork which has been in touch with the wine during aging and it is actually a quality sign which shows that the wine has not been overfiltered which can strip away some of the flavors.
When we crush and press the red grapes there are small particles from skins and seeds which form a sediment at the bottom of the barrels as the wine is aging. Four times during the two years of barrel aging we gently remove the wine and clean out the sediment at the bottom and then refill the barrels again. This process means that we need very little filtering at the end, leaving most flavors in the wine intact. However, after bottling, over the years as the wine ages in the bottle, microscopic sediment will form in the bottle. As a winery we store wine in cases and they are always oriented upside down to keep the cork in contact with the wine and not drying out. This means that most of the sediment will form at the bottom of the cork. If you store the wine lying on its side, which is case with most wine racks, you will find it along the inside of the glass. (The only way you should not store the bottle is cork up.)
You may also find small crystals, like little sugar grains, at the end of the cork which are tartrates. They are formed by tartaric acid and potassium which naturally exist in grapes and come together to form tartrates as the wine ages. Sometimes you will find them in young wine if it has been refrigerated and there is more tartaric acid in white wines. The crystals have no flavor impact and are harmless. However, most wineries will cold stabilize white wine so no crystals form at the bottom of the bottle which can scare consumers. The process involves chilling the wine in the tank so the crystals form and can be removed before bottling.
If you open bottles which have a lot of sediment you can simply decant the wine to leave the sediment at the bottom. If there is a lot of sediment on the cork you can also gently wipe around the opening of the bottle so sediment does not travel with the wine into the glass.