since they often have a "hot spot". Higher quality, mass-produced woks are made of heavy gauge (14-gauge or thicker) steel, and are either machine-hammered or made of spun steel. The best quality woks are almost always hand-made, being pounded into shape by hand ("hand hammered") from two or more sheets of carbon steel which are shaped into final form by a ring-forming or hand-forging process. Cast iron Two types of cast iron woks can be found in the market. Chinese-made cast iron woks are very thin (), weighing only a little more than a carbon steel wok of similar size, while cast iron woks typically produced in the West tend to be much thicker (), and very heavy. Because of the thickness of the cast iron, Western-style cast iron woks take much longer to bring up to cooking temperature, and their weight also makes stir-frying and techniques difficult. Cast iron woks form a more stable carbonized layer of seasoning which makes it less prone to food sticking on the pan. While cast iron woks are superior to carbon steel woks in heat retention and uniform heat distribution, they respond slowly to heat adjustments and are slow to cool once taken off the fire. Because of this, food cooked in a cast iron wok must be promptly removed from the wok as soon as it is done to prevent overcooking. Chinese-style cast iron woks, although relatively light, are fragile and are prone to shattering if dropped or mishandled. Non-stick Steel woks coated with non-stick coatings such as PFA and Teflon, a development originated in Western countries, are now popular in Asia as well. These woks cannot be used with metal utensils, and foods cooked in non-stick woks tend to retain juices instead of browning in the pan. As they necessarily lack the carbonizing or seasoning of the classic steel or iron wok, non-stick woks do not impart the distinctive taste or sensation of . The newest nonstick coatings will withstand temperatures of up to , sufficient for stir-frying. Woks are also now being introduced with clad or five-layer construction, which sandwich a thick layer of aluminum or copper between two sheets of stainless steel. Clad woks can cost five to ten times the price of a traditional carbon steel or cast-iron wok, yet cook no better; for this reason they are not used in most professional restaurant kitchens. Clad woks are