Madeira is made primarily from the Tinta Negra Mole grape with lesser amounts of Sercial, Verdelho, Bual, and Malvasia. The methods for making Madeira vary widely. The Estufa system produces most of the Madeira today. The Estufa system involves heating the wine to 50°C (122°F) for a minimum of 3 months. Some high quality producers age in barrels with natural heating and cooling cycles for 20 years and occasionaly up to 100 years. Madeira is known for very high acidity and the ability to be aged almost indefinately. Alcohol contents tend to range between 17 and 20 percent alcohol.