Is Fire King Microwave Safe
What to know

Is Fire King Microwave Safe?

Is Fire-King or Fire King safe for microwave use? Before anything else, let’s discuss what it is. This glassware company brand that makes dishes and mugs uses glass similar to Pyrex. It used to be made of low-expansion borosilicate glass, making it oven-safe rather than microwave-safe.

Since then, it’s been composed of soda-lime silicate glass. So this begs the question: Is fire king microwave safe? Keep on reading to find out the full answer.

Is Fire-King Microwave-Safe?

For the most part, you can safely microwave beverages on a Fire-King mug. The mug should not be put inside a dishwasher though. The low-expansion borosilicate glass can take high oven temperatures, leading to thermal shock resistance.

The soda-lime-silicate glass has been designed with microwaves in mind (in the 1940s, microwaves didn’t exist), so they have a measure of microwave-safeness to them. Neither could tolerate the rough-and-tumble ruggedness of a dishwasher machine though.

Fire King Blue Cornflower 12 oz
Fire King Blue Cornflower 12 oz

Microwave-Safe Markings

None of the earlier borosilicate glass pieces have a microwave-safe marking to them. They mainly feature heat-proof abilities and require standard thermal shock precautions (don’t put the hot mug on a sink full of cold water).

The breaking point of this brand of glassware dish mostly depends on heat resistance, thermal shock (rapid increase or decrease in temperature), and blunt-force trauma.

Long story short, many and most Fire-King glassware pieces are microwave-safe. Some, however, are more microwave-safe than others. You need to test them to be sure using the microwaved ice water test.

 See more: What Can You Put in a Microwave?

Which Fire-King Glassware is More Microwave-Safe?

Which Fire-King glassware is more microwave-safe? Low-expansion borosilicate glass or soda-lime-silicate glass? The current material used for Fire-King mugs, dishes, and plates—soda-lime-silicate glass—is of lower grade than borosilicate glass, which was what old Fire-King glassware was made of.

Borosilicate glass has greater resistance to thermal shock because of its low CTE (Coefficient for Thermal Expansion). All the things that made it ideal for oven use also make it ideal for microwave use. It superbly deals with high temperatures.

However, even though soda-lime glass has high CTE, Fire-King does manufacture microwave-safe soda lime glassware. However, borosilicate simply features superior thermal shock resistance when all is said and done.

Why is Fire-King Microwave-Safe but Not Dishwasher-Safe?

The oven-safeness of even early Fire-King mugs and dishes give them a measure of microwave-safeness to them. Glass, for the most part, reacts to microwave radiation like transparent glass would to sunlight—they simply pass through the material.

The older borosilicate glass Fire-King offers heat-proof features, like the Blue Bubble. However, non-Fire-King Anchor-Hocking patterns like Charm aren’t heat-proof and thusly aren’t microwave-safe. If you have Fire-King mugs with gold trim, do not microwave them for fear of “arcing”.

The Perils of Washing Fire-King with a Dishwasher

Fire-King mugs and dishes could break upon impact with other dishes when washed inside the dishwasher. Furthermore, the original luster of the glassware will get a dishwasher haze because the machine removes a thin layer of glass on them.

The value of the Fire-King mug tends to drop when washed on the dishwasher regularly. Hand-washing helps retain Fire-King glassware luster for much longer compared to the permanent and irreversible consequences of dishwashing.

Antique Fire-King glassware pieces with seemingly sandblasted surfaces were most probably washed with the dishwasher. Don’t wash inside the dishwasher hand-painted Gay Fad or Peach Luster pieces as well.

How Do You Tell the Age of a Fire-King Piece?

The microwave-safeness of your Fire-King mug depends on whether it’s a new or old Fire-King. This mug has been around since World War II. To know when a Fire-King got made, it should have a mark on it. However, some lack this mark. Over time, the marks changed.

Collectors also depend on such markings to verify Fire-King mug or dish authenticity. The markings give them an idea of when they were made, how old they are, and if they’re the genuine article Fire-King brand glassware.

The list of Fire-King trademarks according to date of manufacture includes the following:

Trade MarkDate Range
FIRE-KING  (Block Lettering)1942-1949
OVEN FIRE-KING WARE (Block or Script Lettering)Mid 1940s to Late 1940s
OVEN Fire-King WARE MADE in U.S.A. (“Fire-King” is in Script Lettering) 1951-1960
ANCHOR HOCKING OVEN Fire-King WARE MADE IN U.S.A. (“Fire-King” is in Script Lettering)1960 to Late 1960s
ANCHOR HOCKING OVEN Fire-King DINNERWARE MADE IN U.S.A. (“Fire-King” is in Script Lettering)Late 1960s to Early 1970s
ANCHOR HOCKING OVEN Fire-King OVEN-PROOF MADE IN U.S.A. (“Fire-King” is in Script Lettering)Mid to Late 1970s
ANCHOR HOCKING OVEN Fire-King Suburbia OVEN-PROOF MADE IN U.S.A. (“Fire-King” is in Script Lettering)Late 1970s

Fire-King also marked the words “OVEN PROOF” or “HEAT PROOF” on some pieces, while other pieces have the anchor marking alone. The style of the anchor logo can also assist in establishing which era of Fire-King glassware you got on hand.

If Your Fire-King Lacks Markings, Is It Fake?

Not necessarily. Any highly produced pattern can undergo multiple molds. As these molds gather cumulative glass layers until they couldn’t be used, you replace them with brand new molds. The new molds could then have the logos and anchor changed for a new brand line.

Therefore, if your Fire-King lacks certain markings, it might be because it got molded by a mold nearing the end of its usefulness. Not all molds with a certain mark will wear out at the same period of time. Two or more kinds could be used simultaneously for certain brief instances.

Because of this, Fire-King makers switched to foil labels instead of depending on mold marks. The foil labels tend to be removed by customers as time passed by, leading to unmarked Fire-King products. To collectors, these unmarked pieces are no less collectible or valuable than pieces with imprinted marks.

What Else Do You Need?

Test the glass to see if it features full microwave-safeness or partial microwave-safeness. A mug or dish that gets warm when microwaved has partial microwave-safeness while a mug or dish that remains cold after microwaving offers full microwave-safeness.

Just microwave at full power a mug of ice-cold water on these mugs until they end up boiling. If the mug remains cold it’s fully microwave-safe. If the mug becomes warm it’s partially microwave-safe. If the mug ends up as hot as the water then it’s not microwave-safe.


  1. FAQs for Fire-King Collectors“,, August 21, 2021

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