As a result of the increase in microwave oven sales, microwave cookware has emerged in a variety of materials-glass, glass ceramic, plastic and paper. Some cookware specifically for microwave ovens can also be refrigerated, frozen and used in conventional ovens.

Microwave cooking requires the use of microwave-safe cookware. Some kinds of cookware can become very hot in the microwave and pose a hazard. Generally, microwave-safe cookware does not heat up in the microwave as the cooking energy is absorbed by the food..

A simple test to determine if a dish is microwave-safe is to place the dish in question in the microwave along with a cup of cold water in a known microwave-safe item. Microwave on high (100 percent) power for one minute. If the water has heated and the dish has remained cool, it's microwave safe. If the dish tested has gotten warm or hot, it should not be used in the microwave oven.

A container used in microwave cooking must allow microwaves to pass through both it and the food. Contrary to popular belief, some metal can be used in microwave cooking; its reflective properties can even help protect food which might overheat in some areas.

Aluminum foil for shielding, small skewers and shallow food convenience trays can be used in microwave ovens; however, metal should be kept at least 1" away from oven walls, and deep trays and metal pans aren't suitable. Foil-lined cartons shield food completely, and don't heat food at all.

Generally speaking, shallow containers produce better results than deep ones; round shapes tend to be better than square or rectangular ones. Microwaves travel in straight lines, bouncing around the oven in irregular patterns. Therefore, sharp corners allow more exposure to microwave energy so the food in these areas dries out before the center is cooked.

Plastics for the most part are transparent to microwave energy and are ideal for microwave use. A variety of plastics is available, and the quality of the plastic in microwave ovenware has much to do with its safety. "Engineered" plastic (heavy-duty industrial grade) is not only more expensive than many plastics, it's likely to damage a microwave oven.

The Society of Plastics Industry is developing test methods for manufacturers of plastic cookware to use as guidelines in evaluating the durability and safety of their products. Newly developed, heavy-duty plastic microwave cookware that is not harmful to microwaves comes in a variety of shapes and sizes-from casserole dishes to muffin pans. Some of this cookware also can be used in conventional ovens at low temperatures.

In general, plastics are stain resistant, break resistant and freezable, but the combined production of steam and hot fans in microwave ovens can distort some of the less-durable plastics.

Those labeled to withstand boiling water, or as dishwasher safe, are often recommended for microwave use because they can take the heat of food for short reheating and thawing periods without melting or distorting. For true cooking, exotic resins like PBT, TPX, etc., have 350 degrees to 450 degrees melting points.

Melamine dishes are usually limited to one or two minutes of cooking time by most oven manufacturers, if they're recommended at all, because they can become very hot and scorch or crack.

Wood and natural materials such as straw are usually limited to one or two minutes of cooking time by most manufacturers of microwave ovens. The inherent or soaked-up moisture and fats in wood can absorb the microwaves and cause the wood to heat, resulting in drying, cracking or scorching.

Ceramics, including pottery and earthen-ware, are suitable for use in microwave ovens, but oven manufacturers recommend that they be tested first. Some ingredients that absorb microwave energy and heat rapidly to a high temperature are present in some ceramic dishes. Large amounts of these particles can result in the dish overheating and breaking.

Glass cookware is identified as heat resistant or non-heat resistant, while most glass-ceramic cookware is classified as glazed or unglazed. Most manufacturers recommend the use of heat-resistant glassware or glass-ceramic cookware for microwave cooking.

Non-heat-resistant glass dishes are not treated to withstand the extreme and uneven heat normal in microwave cooking; i.e., the glass remains cool while food gets hot; the hot food then transfers heat at the points where it touches the glass, causing uneven heating in the glass that leads to breakage for non-heat-resistant glasses.

Glazed glass-ceramic dishes are not recommended for microwave oven use. The glazes contain relatively high percentages of ingredients which absorb microwave energy, causing the dishes to heat rapidly to high temperatures. This may result in breakage or could cause burns or spills if they are picked up without potholders or oven mitts by someone not expecting the dish or cup itself to be hot.

Unglazed glass-ceramic and heat-resistant ovenware is highly recommended for use by both ovenware and microwave-oven manufacturers because they are non-porous and cannot absorb moisture of food.