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Iwaki Heat-Resistant Glass Tableware Handling Instructions

The products we used:

Tips for Long-Lasting Use of Iwaki Glass Products

heat resistant glassware bowls from iwaki

Glass products, including heat-resistant glass, are prone to breakage due to tensile stress. To ensure long-term, safe use of glass products, it is crucial to avoid causing scratches or damage by striking them with metal objects, knocking them against hard materials like ceramics, or scrubbing them with abrasive sponges.

Like most substances, glass expands when heated and contracts when cooled. Thermal breakage typically occurs due to rapid cooling or rapid heating. Heat-resistant glass is designed to minimize thermal expansion and contraction, reducing the associated stresses and making it more heat-resistant.

When glass breaks, it always starts from a scratch or imperfection on the surface subjected to tensile stress. The deeper the scratch, the less force is required for breakage. Over time and repeated use, scratches tend to grow deeper, weakening the glass's strength.


heat resistant glassware from iwaki

Product Name: Iwaki Heat-Resistant Glass Utensils Usage Categories: Oven, Microwave, or Hot Water (varies by product) Thermal Shock Resistance: 120°C / 248°F

Applications and Characteristics

Heat-Resistant glass is commonly used in the following applications:

  1. Oven-safe cookware
  2. Microwave-safe cookware
  3. Coffee pots
  4. Laboratory equipment

Heat-Resistant glass is used in those applications because it has the following characteristics:

  1. Low thermal expansion
  2. Heat and acid resistant
  3. Low thermal conductivity
  4. Low electric induction dissipation


Warning: Failure to follow these instructions may result in burns or injury. Please carefully read and follow them for safe, long-term use.

Before Use

inspecting the bowl before use
  • Inspect for cracks, chips, or damage along rims and pour spouts before use (to prevent injury during washing).
  • Wash new glass tableware with mild detergent before initial use, as they are not airtight.

Handling Damaged/Scratched Items

  • Do not use glass tableware with cracks, chips, or deep scratches, as it may break unexpectedly. Discard such items.
  • Exercise caution with long-used items prone to damage.
  • Glass tableware can break easily, so handle with care during washing and use.
  • Do not twist wash by grasping the rim and applying inward force, as this may cause breakage and injury.
  • Broken glass creates sharp fragments that can cause injury. Handle with extreme care.
  • Do not strike or scrape with hard objects like spoons, as this can scratch and lead to breakage.

Washing Precautions

washing heat resistant glassware from iwaki
  • Use a soft sponge with dish soap for washing. Avoid abrasive scrub pads, metal scouring pads, or coarse cleansers, as these can scratch and cause breakage.
  • For heavy soiling, use diluted household bleach, following all handling instructions. Rinse thoroughly after using bleach.

Other Handling Notes

  • Instruct children on proper handling precautions if they will be using the tableware.
  • Do not include with recycled glass bottles when discarding, as the glass composition differs. Follow municipal disposal guidelines.

Heating Precautions (Usage Category Specific)

Oven Use

  • Never heat directly over flame, as this may cause breakage or burns.
  • Do not rapidly cool during oven heating. Wipe away moisture, and avoid adding cold water partway through cooking. Do not place hot glass on wet surfaces.
  • Do not place frozen glass tableware or frozen foods directly in preheated oven (defrost first).
  • In toaster ovens, ensure containers are 3-4 cm from heating element.
  • Only use regular ovens or toaster ovens for heating (no other heating appliances).
  • Do not heat empty or with very little contents.
  • Use oven mitts to handle the entire hot container after heating.
  • Be cautious of weight when placing in/removing from toaster ovens to prevent tipping.

Microwave Use

  • Never heat directly over flame, as this may cause breakage or burns.
  • Do not heat empty or with very little contents.
  • Do not make caramel sauce in the microwave, as localized hotspots can cause breakage.
  • Only use microwaves for heating (no other heating appliances).
  • Use oven mitts to handle the entire hot container after heating.
  • Avoid overheating water, as it can violently boil over when removed (superheating).

Hot Water Use

  • Do not heat, cook, or heat empty using direct flame, oven, toaster oven, microwave, etc., as this may cause breakage or burns.

Frequently Asked Questions

  1. Q: It says Thermal Shock Resistance: 120°C / 248°F, but does this mean oven-safe items can only withstand up to 120°C / 248°F? A: Thermal Shock Resistance is different from Heat Resistance. The thermal shock resistance refers to the temperature difference the glass can withstand without breaking when a heated piece is rapidly cooled by immersion in water. For Iwaki's oven-safe glass, you can safely use it in a household oven (around 300-350°C / 572-662°F). However, be very careful when handling heated heat-resistant glass, as placing it on a wet surface or holding it with a damp cloth can cause rapid, localized cooling beyond the thermal shock resistance temperature, leading to breakage.

  2. Q: Is heat-resistant glass less prone to breaking? A: Heat-resistant glass is designed to withstand rapid temperature changes like pouring hot water, but it is not the same as tempered glass, which is resistant to impacts from knocking or dropping. Like regular glass tableware, heat-resistant glass requires careful handling to avoid breakage from impacts.

  3. Q: Can I put it in the freezer? What foods can be frozen? A: You can put empty glass containers in the freezer without any problem. However, if you put food containing a lot of moisture in them, the volume will expand during the freezing process, and the glass may break. If you put liquids like water in them, even if it's only half full, the liquid will expand in all 360 degrees, causing the glass to break. Raw meat, fish, boiled vegetables with the moisture removed, and stews (with as little liquid as possible) can be frozen. Rice can be frozen, but since it contains a lot of moisture, it will expand during the freezing process, so an extra step is required. While the rice is still hot, lightly form it into rice ball shapes before putting it in. If you use a container with a lid, put the hot rice in, put the lid on, and shake it well left, right, up, and down, so that the rice gathers in the center, creating a space between the rice and the glass. Freeze it in this state.

  4. Q: Can I put something from the freezer straight into the microwave? A: Yes, that's fine. Unlike oven heating, microwave heating only heats the food itself. The container will also get hot, but that's because the heat from the food is transferred to it. There will be no partial temperature difference.

  5. Q: Is it okay to store things with a strong vinegar or salt content, like pickled plums? A: Heat-resistant glass is resistant to acids and salt, so there is no problem. However, for some products, the plastic lids may become deformed by substances contained in citrus fruits like oranges and lemons, so please check the instruction manual if you plan to store citrus fruits.

  6. Q: Can I use glass in a steamer? A: You can use it in a steamer, but the plastic lid may degrade faster, so please use it without the lid.

  7. Q: Can I use ovenware in a toaster oven? A: You can use ovenware in a toaster oven, but please make sure it is 3-4 cm away from the heating source. If it's too close to the heat source, it will be like direct heat, which could cause the glass to break.

  8. Q: Can I use it on an induction cooktop or a fish grill? A: Glass does not react with induction cooktops. Since you cannot use it over direct heat, you cannot use it on a fish grill, but you can use it with the grill function of a microwave.

  9. Q: Can I disinfect it with hot water? A: You can disinfect the glass itself by pouring boiling water over it. As for any included components, please check the instruction manual.

  10. Q: It broke when I was making caramel sauce in the microwave. A: You cannot make caramel sauce in the microwave using heat-resistant glass. The high temperature (160-200°C / 320-392°F) of the caramel sauce inside the container causes a temperature difference between the areas in direct contact with the sauce and those not in contact, resulting in thermal breakage.

  11. Q: The lid was microwave-safe, but it melted when I put it in the microwave. Why did this happen? A: There are two possible reasons:

  12. If you used a mode other than the microwave mode, or an automatic mode, it may have reached a high temperature and caused the lid to melt.

  13. The ingredients or oil in the food you heated may have caused abnormal heating, raising the temperature and causing the lid to deform. Please read the microwave's instruction manual carefully before use.

  14. Q: I put a heat-resistant glass container in the microwave (oven) and it broke... A: If the glass gets scratched during use, those scratches can cause the glass to break when heated. Please use it carefully according to the instruction manual to avoid scratches as much as possible.

  15. Q: I took a frozen food item out of the freezer, put it in an oven-safe glass container, and put it in a 180°C / 356°F oven, but the glass broke. I had just taken the glass out of the cupboard, so why did it break? A: Even though the glass was at room temperature, the area where the frozen food was placed remained cold, taking longer to heat up. However, the areas not in contact with the food heated up quickly, creating a temperature difference greater than the glass's heat resistance, causing it to break.

The products we used: