Heating glassware is an important topic as heat can change the volume of the glassware if the temperature used is too high. The maximum temperature for heating the glass depends on the specifications and what type of glass is used. For example, borosilicate glassware provides a lower coefficient of expansion than other glass types such as soda lime glass. Another note is that not all glassware is manufactured to the same quality standards. The quality of raw materials used and other key factors such as consistency of glass thickness can vary, even with premium laboratory and scientific glassware brands.
Here are some tips for safely heating glassware in the lab:
1) Factor in expansion and contraction at high temperatures
Borosilicate glass has a low coefficient of expansion (3.3×10-6K-1) meaning is has very little expansion or contraction when heated or cooled, however, care is still required even with this glass type.
Once the temperature exceeds 150oC, extra care is needed to ensure heating and then cooling of the glassware is achieved slowly and uniformly. It is important to factor in the necessary equipment, conditions and time needed for gradual increases and then decreases of temperature.
2) Remove the risk of thermal shock
Any sudden changes, including excessive temperatures, should be avoided as thermal shock from sudden heating/cooling can cause the glass to crack or break. The glass should be heated gently and gradually, including at the beginning of your process, and allow hot glassware to cool gradually – away from cold draughts.
3) Avoid hotspots by distributing heat evenly
Concentrated/direct heat on one part of the glass can cause hotspots to occur, these should be avoided as different rates of heating can cause stress that weakens the glass and causes breakages. If you are using a Bunsen burner, employ a soft flame and use a wire gauze with a ceramic centre to diffuse the flame, to help distribute the heat applied.
Using a hotplate also helps to ensure effective heat distribution, it is important to ensure the top plate is larger than the base of the vessel being heated. This will enable even heat distribution of heat through the base of the glassware, reducing the possibility of glass breaking due to hotspots. Always warm up the glassware to ambient temperature before placing it on the hotplate, as cold containers can be subject to thermal shock.
4) Microwave with caution
Not all laboratory glassware can be used safely in a microwave. For example, soda lime products are unable to endure sudden temperature changes, but borosilicate glass is microwave safe. However, it is important to ensure that the vessel holds microwave absorbing material before placing it in the oven.
You also need to check for any parts or accessories attached to the glass to ensure the material they are made from can be safely microwaved. Some products utilise plastic screw caps and connectors, these should be manufactured from polypropylene or PTFE to be microwave safe.
5) Autoclave with care to prevent damage ahead of future heating
Most laboratory glassware can be safely autoclaved. There are a few tips to keep in mind to avoid damaging glassware. Firstly, always loosen off any screw caps before starting the process. Autoclavable glassware with a tightly screwed cap can result in pressure differences which can damage the container, and result in damage either there and then, or when heating the glassware in the future. Alternatively, using a single port connector cap with a sterile venting filter or a sterile venting membrane screw cap provides an automatic sterile gas exchange. This allows for safe pressure equalisation during the autoclave cycle whilst also ensuring the sterility of the bottle contents.
Secondly, avoid overloading the autoclave. This will ensure enough space is left between items to circulate the high-pressure steam, and reduce the risk of damage that might later make the glassware unsuitable for heating.
All blog information was provided courtesy of DWK
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