Sometimes you will find crystals in a bottle of wine. They look like sugar crystals, but they do not dissolve. Those crystals are called WEINSTEIN or "wine stones" and consist of potassium-bitartrate.
When you know about it and recognize these crystals, do not mistake them for sugar or such, it tells you that the wine has been made from ripe grapes, that had sufficient acidity and minerals to have these crystals form. As the grapes ripen on the vine they gather minerals such as potassium from the soil. The riper the grape, the larger the percent of tartaric acid ("Weinsäure"). Weinstein is the joining of potassium and tartaric acid, which partly form these insoluble crystals in the wine. The crystals form under rapid lowering of the temperature, i.e. refrigeration before serving.
Weinstein or potassium-bi-tartrate affects in no way the taste, clarity or color of the wine. It states that the grapes first of all had fully ripened and second of all the grapes have an accompanying acidity that keeps the wine fresh and long lived.
How to serve such a wine?
When such a wine is served, it should be poured very carefully. The natural weight of the crystals holds them in the bottle when poured slowly into the glass so that no tartar crystals reach the glass. It is not suggested to decant such a wine, since a white wine loses much of its aroma and bouquet when it comes into contact with air. The crystals in no way impair the quality or adversely affect the taste, the clear lustre or the bright color of the wine. These crystals are also formed in red wine, but one would mostly not notice.
Decanting is recommended for wines with sediment. Often an older wine will throw particulate matter such as pigments or tartrate crystals. A younger wine may have tartrate crystals. First agitate the bottle to loosen any sediment that is clinging to the side. Then allow the bottle to rest upright until the sediment has settled to the bottom. The time depends on the type of sediment, with tartrate crystals settling rather rapidly (half and hour) and pigments requiring much longer (several days). You may want to allow two weeks to be on the safe side. If you wait longer, you will not hurt the wine.
Remove the capsule and cork carefully without tilting the bottle. Use a cloth to wipe off the rim of the bottle. Find a bright light or window. Position a carafe or a row of glasses so that they are handy. Hold the bottle at eye level against the light so that you are looking through the wine. Lift the carafe or a glass to the neck of the bottle. Tilt the bottle slowly and begin pouring the wine slowly and continuously. Do not stop pouring until you see the sediment reach the neck of the bottle, because the sediment will be agitated as soon as you move the bottle toward its upright position.