Glass coffee mugs are a popular choice for enjoying hot beverages because they let you see the color of your drink. However, glass can crack or shatter when exposed to sudden temperature changes, a phenomenon known as thermal shock. Understanding why this happens and how to handle glass mugs safely helps you avoid accidents and extend the life of your drinkware.
What is thermal shock?
According to a 2019 Iowa State University Extension blog about glass kitchenware, thermal shock occurs when an object abruptly goes through a drastic temperature change, causing it to fracture as it expands or contracts. The blog explains that under the right circumstances, glass bakeware and kitchenware can shatter, crack, split, or even explode. The same principle applies to glass coffee mugs: pouring boiling water into a cold mug or setting a hot mug on a wet countertop can create enough stress to break the glass.
Types of glass used in mugs
The Iowa State University blog describes two main types of glass found in kitchenware: borosilicate glass and tempered soda-lime glass. Borosilicate glass has low thermal expansion, which means it handles rapid temperature changes better. Tempered soda-lime glass is more common in modern glassware sold in the United States, and while it withstands drops better, it does not handle heat as well as borosilicate glass. The blog notes that with some caution, tempered soda-lime glass withstands thermal shock reasonably well.
You can often identify the glass type by looking at the markings on the underside of the mug. The Iowa State blog explains that older PYREX® (all uppercase letters with a trademark notice) was made from borosilicate glass, while pyrex® (all lowercase letters) indicates tempered soda-lime glass. When shopping for glass mugs, check for any labeling that indicates the glass composition, and keep in mind that not all glass drinkware is designed for extreme temperature swings.
Safe handling to prevent cracking
The Iowa State blog shares several precautions originally issued by Consumer Reports to minimize the chance of glassware shattering. These guidelines, while written for bakeware, are useful for glass coffee mugs as well. Always place hot glassware on a dry, cloth potholder or towel. Never place hot glassware directly on a countertop, metal surface, damp towel, in the sink, or on a cold or wet surface. Do not add liquid to hot glassware, and avoid taking a mug directly from the freezer to a hot environment or vice versa. Inspect your mugs for chips, cracks, and scratches, and discard any with such damage.
Manufacturers of specific products also provide care instructions. For example, CorningWare’s stoneware Meal Mug product page warns that when heated or cooled too rapidly, stoneware can break. Their instructions say not to place hot stoneware on a wet or cold surface, not to handle it with a wet or cold cloth, and not to place it on a metal stove grate. While this advice is for stoneware, not glass, it reinforces the general principle: avoid sudden temperature changes. Always read and follow the care instructions that come with your own mug. If no instructions are available, the precautions from the Iowa State blog offer a solid starting point for safe use. For more on caring for different mug materials, see our care by material guide.
Microwave use and glass mugs
The U.S. Food and Drug Administration (FDA) explains that microwaves pass through glass, paper, plastic, and similar materials. This makes glass a convenient choice for reheating coffee. However, the FDA warns that glass containers can become hot from the heat of the food or liquid inside, so always use a dry potholder when removing a mug from the microwave.
A specific hazard the FDA highlights is superheated water. Water heated past its boiling temperature in a clean cup may not appear to be boiling, but a slight disturbance, such as picking up the cup or adding instant coffee, can cause a violent eruption of boiling water. The FDA states that adding substances such as instant coffee or sugar before heating greatly reduces this risk. Users should also follow the microwave oven’s instruction manual and not exceed recommended heating times. If you frequently reheat coffee, our article on microwave compatibility offers additional guidance.
Scald prevention with hot drinks
Hot coffee can cause serious burns. The American Burn Association’s scald prevention fact sheet advises keeping hot drinks away from the table and counter edges and using travel mugs with tight-fitting lids for all hot drinks. These simple steps help protect children and adults from accidental spills. When choosing a travel mug, look for a secure lid that won’t pop open if the mug is knocked over.
A quick safety checklist
Keep these points in mind each time you use a glass coffee mug:
- Check the mug for chips or cracks before use.
- Preheat the mug gradually if you plan to pour in very hot liquid, or start with warm water.
- Always set a hot mug on a dry cloth, potholder, or cooling rack, never on a cold or wet surface.
- Do not add cold liquid or ice to a hot mug.
- When microwaving water, add instant coffee or sugar before heating, as the FDA suggests, to reduce superheating risk.
- Keep filled mugs away from counter edges, and consider a travel mug with a tight lid when moving around.
- Follow any specific care instructions from the mug’s manufacturer.
For a broader look at how glass compares to other materials, visit our coffee mug materials comparison. You can also explore the full range of mug types to find the best fit for your routine.
Frequently asked questions
Can I pour boiling water directly into a glass coffee mug?
It depends on the mug’s glass type and its condition. The Iowa State University blog explains that borosilicate glass handles rapid temperature changes better than tempered soda-lime glass. Even with borosilicate, it is safer to warm the mug first with a little hot water, then add the boiling water. Always check for chips or cracks, as damaged glass is more likely to break.
How can I tell if my glass mug is borosilicate or soda-lime glass?
Look for markings on the bottom of the mug. The Iowa State blog notes that PYREX® in all uppercase letters with a trademark symbol often indicates borosilicate glass, while pyrex® in lowercase letters typically signals tempered soda-lime glass. If there are no markings, you may need to consult the manufacturer’s product information.
Is it safe to microwave a glass mug with a metal rim?
The FDA states that metal pans or aluminum foil should generally not be used in a microwave oven because microwaves are reflected off these materials, which can cause uneven cooking and possibly damage the oven. A metal rim on a mug could create similar issues. Check the mug’s labeling or the manufacturer’s instructions to see if it is marked microwave-safe. When in doubt, transfer your coffee to a mug you know is microwave-compatible.
For more tips on selecting and maintaining glass drinkware, browse our glass mugs category page.
