Which products made by Corning are safe for microwave use?

Many products still in use today were manufactured before the advent of the microwave oven and they bear no warnings or approvals for microwave use.  Even some newer products, like clear Pyrex and Corelle, are definitely microwave-safe, but their backstamps neglect to state this specifically.

So it is important to recognize which items can be microwaved by understanding Corning's different glass formulas.  The consequences of making a mistake might not lead to immediate explosions, but there are potential health concerns and the possibility of damage to the dish and the oven too. 

If doubts remain about microwaving older pieces, the best option would be using newer items which are acknowledged to be microwave-safe, either on the backstamp or on the original packaging or label.  These items have been thoroughly tested already by the manufacturer.  (also see: Microwave Test)

For one reason or another, the following items should NOT be used in the microwave:

- Pyrex or Corning Ware with gold, silver or platinum decoration.  Metallic decorations will attract sparks and the pattern can be damaged permanently with burn marks.  It is possible that arcing also can lead to a fire or otherwise damage the oven if it is allowed to continue.  There is no method of testing whether the design will cause problems.  If it has a highly reflective metallic shine, it would be best not to microwave it.

(Photo: Platinum-banded cream & sugar set from the late 1960s.  Gold designs can be found on both clear and opal Pyrex, but silver and platinum patterns appear on clear Pyrex only.)

Some French White pieces have shiny gold-coloured trim, and they are the exception to this rule.  These items were made more recently and the original packaging states that they are microwave-safe.  The trim looks different than real gold leaf; it is a brassy or copper-toned shade of gold.

- Centura, a barium-aluminosilicate (celsian) glass-ceramic, attracts microwave energy away from the food and can become extremely hot during normal cooking times.  It may shatter if it is cooled too quickly, and there is also a possibility the metallic content can be released into the food.  The Centura formula was used only for dinnerware, not cookware.  It is not always simple to identify it since it was marked with many different brand names, and there are also pieces marked Centura which are not made of Centura.  There is a paragraph below for that information.

Avocado Round Corning Ware: like Buffet Servers, the knobs are attached to the lids with a metal pin.  (Image from 1968 advertisement).

- Corning Ware solid glass-ceramic lids with screwed-on knobs.  Round cookware with a Buffet Server shape often came with white or solid coloured or even patterned lids.  Some of these are made of Centura, but most are made of Corning Ware.  Even if a lid can be identified as Corning Ware, the method of attaching the knob prevents its use in the microwave.  Knobs are attached with a metal pin or screw easily visible on the underside.  The Corning Ware casserole is safe for microwaving without the lid.
              
- Corning Ware Rangetoppers have an aluminum layer bonded to the bottom for improved stovetop performance.  A small number of Rangetoppers have completely white bottoms with no visible metal, but there might be a metal base concealed within the glass-ceramic, it is difficult to tell.  In any case, both types of Rangetoppers are marked "Not for microwave use", and product warnings should never be ignored.

(Photo:  N-2½ Rangetoppers saucepan.  The bottom of this one is white and shiny.) 

Rangetopper N-10 skillet with visible aluminum bottom.

- Older Pyrex with gas bubbles or bits of debris may shatter with microwave energy.  The pockets of gas might react to the microwaves and expand rapidly, and any ash or grit within the glass also may attract uneven heating or expansion.  The Pyrex casserole below has two flaws; black spots are another common flaw as well.  Glass forming technology has come a long way since 1915, and these imperfections are commonplace in older glass.

- Depression Glass by MacBeth-Evans & Corning includes transparent glass and opal glass.  Like older Pyrex, these pieces can have bubbles and impurities.  But in addition to this, different minerals are present in these formulas to achieve different colours and to improve clarity.  Items with coloured decorations are embellished with enamels which have unknown additives.  There is no such thing as "just glass", and it is difficult to predict how very old glass reacts to microwaves.  Surely there is no crucial reason for glass of this age to be put through its paces in the microwave when there are so many safe alternatives easily at hand.

(Photo: MacBeth-Evans S-Pattern sugar bowl with a bubble.)

  
The following items are perfect for the microwave, with certain exceptions:

- Corning Ware cookware is microwaveable, regardless of its age.  It should not have any metal bands, handles or screws attached, no metal bottom, and no metallic decoration.  But French White Gold is safe even with its gold-like trim. 

(Photo: Corning Ware backstamps from the 1970s & early 1980s recommend microwave usage.  Older stamps and newer stamps fail to mention it, but these items are microwaveable too.)

Corning Ware cookware made to match Centura Tableware patterns often is labelled as Centura too, but the 'glowing flame' symbol also found on these pieces shows that they are stove-top ware and are not made of Centura at all.  Despite the misleading brand name, they are Corning Ware.

- Opal Pyrex and clear Pyrex are microwave-safe, provided there is no gold, silver or platinum pattern.  But older Pyrex pieces with gas bubbles or sediment are best kept out of the microwave.

(Photo: Pyrex backstamps from the mid to late 1970s & 1980s recommend microwave usage.  Older stamps and newer stamps fail to mention it, but these items are microwaveable too.)

Scientific studies of microwave cooking conducted in the 1940s used clear Pyrex and determined that it was appropriate for this use.  Similar studies in the early 1960s also used opal Pyrex in this manner.

- Flameware might be safe, but it depends on whether a particular item passes a microwave test.  Older items with a darker tint do gain a little warmth when tested, but newer pieces with a lighter tint remain cool under the same conditions.  The intensity of the tint might not be the sole reason for an item passing or failing the test, but it serves as an indicator of the age of the glass.

Since metal handles and bands do not belong in the microwave, the only items that are practical for this type of use are pans with detachable handles, and platters.  Flameware also should be checked for gas bubbles and sediment since this product dates to 1936.

Flameware platters (815 & 812) and 7 inch skillets (817).  Flameware was deliberately tinted to differentiate it from ordinary clear Pyrex.  The exact colour varied a lot, but in general, the oldest items have the darkest tints.

- Corelle plates, platters, saucers, bowls, open-handle cups, open-handle creamers and corresponding sugar bowls.  These items are made of Vitrelle, and are microwavable.  Corelle was never made with metallic decoration.

(Photo: Corelle backstamps from the late 1980s to the early 2000s recommend microwave usage.  Older stamps and newer stamps fail to mention it, but these items are microwaveable too.)

The composition of closed-handle cups made to match Corelle should be determined first.  Opal Pyrex, Suprema, Corning Ware, porcelain and stoneware cups are safe, but Centura is the type to be aware of.  See Cup Styles for details on identifying cups. 

- Visions cookware is completely safe for the microwave.

- Modern stoneware branded with 'Corningware' or 'Corelle Coordinates' is microwave-safe.


For the Microwave ONLY:

On the flip-side, although Corning Ware Browning Skillets and Grills are made of real Corning Ware, they cannot be used on the stove-top or in the oven.  Microwave browning products have a grey rectangle of tin-oxide embedded in the base, and this will be damaged if exposed to direct heat.  The damage is visible as a burnt smoky appearance. 

Other Corning Ware items like bacon racks and Fast Food dishes are specially designed for microwave use, but they are not browning utensils either as they have no tin-oxide in their bases.


Corning Ware Microwave Browning Skillet
                         
Browning skillets require specific pre-heating and cooking methods.  Visit Microwave Cooking for One for instructions.


Identifying Centura:

There are just three types of products that are made of Centura and none of them are cookware.  With superlative break-resistance, Centura's glass-ceramic formula is designed specifically for dinnerware; it is not meant for stove-top use like Corning Ware is.  Cookware never was made from Centura glass-ceramic because it does not have the same heat-resistance that Corning Ware does.  Centura would shatter if it was placed on a hot burner.

The Centura formula was used for:  1) closed-handle cups to match Corelle; 2) Centura Tableware; 3) commercial tableware for restaurant use. 

1) Corelle-matching Centura cups generally are identifiable by their 'No Microwave' warning.  The 'Livingware' cups are one exception, and Cup Styles is the place to look for more information about all types of Corelle cups.

                   
2) Centura Tableware is instantly recognizable with the 'Centura by Corning' logo on the backstamp.  Some newer pieces even have a 'No Microwave' warning. 

A major exception to this rule is Corning Ware cookware stamped with the Centura brand name.  These products were made to match the dinnerware, and marked Centura in the interest of brand cohesiveness.  This cookware is most definitely real Corning Ware and it bears a logo proving which composition it actually is, despite the name on the stamp.  But some of the lids for these items have a metal screw on the underside for attaching the decorative knob, making the lid unsuitable for microwaving.

3) Centura restaurant ware was branded as 'Pyroceram Tableware by Corning'.  It has been said elsewhere that the word 'Pyroceram' refers to the Corning Ware formula, but this is only half true, and leads to confusion. 

'Pyroceram' is in fact a vague brand name that Corning used to describe the entire family of glass-ceramics which encompasses many formulations.  Visions, Centura, Suprema and Corning Ware are all glass-ceramics, and all were called Pyroceram by the company at one time or another.  But they undoubtedly have distinct formulations, usages and restrictions.

(Photo: A small restaurant ware ramekin or bouillon cup.  It is identical to a Centura Tableware sugar bowl in both shape and composition.)

Suprema, a microwave-safe glass-ceramic formula, also was used for restaurant ware, and many Suprema pieces were branded as 'Corning Pyroceram' or 'Corning Pyroceram Tableware'.  Determining whether a piece is Suprema or Centura can be done easily by holding a piece up to a bright light and wiggling your fingers behind it.  Suprema does have a slight translucency, while Centura is very dense and light will not pass through it at all.  A microwave test also will prove definitively whether it is a safe formula.
 

Microwave Testing:

This test can be performed to determine if the glass contains an element which attracts microwave energy, and therefore is unsuitable for use, like Centura.

- Fill a known microwave-safe cup or measuring cup with water.  Pyrex is a good choice. 

- Place this in the microwave along with the dish that is to be tested.  Ideally they should not be touching.

- Heat on High for 30 seconds to one minute, whatever interval it normally takes to make hot water.

- If the empty dish is hot, or even warm, it is not suitable for microwave use.  Be cautious, as the dish could be uncomfortably hot too. 

Browning pans will become very hot while empty in the microwave, but this is normal and part of the pre-heating routine.  The tin-oxide rectangle is always on the bottom, so it never touches the food.


Related Articles:

1982 Corning Ware Use & Care
1988 Corning Ware Use & Care
Flameware Use & Care
Corelle Use & Care
Corning Ware Buffet Servers
What is Centura?
What is Suprema?
What are glass-ceramics?
Compare Pyrex & Flameware Platters