Laboratory watch glasses: for which covering, weighing and evaporation applications are they used?

Indispensable on any lab bench, the watch glass is a shallow, concave dish used to cover a beaker, evaporate a solvent, weigh out a solid or dry a small residue. Its simplicity hides real know-how: materials (borosilicate 3.3, quartz, PTFE or plastic), calibrated diameters, polished rims for safety, compatibility with autoclaving and thermal resistance. Choose the right one and it safeguards your procedures, limits contamination and speeds up prep work, from quality control to academic research, via teaching.

What are the different types of laboratory watch glasses, and how do you choose the right one?

First, we distinguish borosilicate glass 3.3 (everyday use): excellent chemical resistance, low expansion, withstands heating cycles and autoclaving. Soda-lime glass is more economical but less resistant to thermal shock: ideal for room-temperature covering and simple weighing. Quartz models tolerate very high temperatures and offer UV transmission useful in photochemistry. In highly corrosive media or in the presence of HF, favour PTFE, which is chemically inert and non-stick. Finally, plastic versions (often polystyrene) are lightweight and disposable for cleanliness-critical areas, but not suitable for heat or aggressive solvents.

Selection criteria:
• Diameter (50 to 200 mm) matched to the vessel; to cover a beaker, aim for 20–30% more than the opening.
• Concavity and thickness for sample stability and robustness.
• Chemical compatibility (avoid any glass/HF contact; avoid organic solvents with plastic).
• Temperature and thermal shock (hotplate, drying, desiccator).
• Sterilisation (autoclave or dry heat) and re-use.
• Finish with polished edges for safety and durability, labelling area if needed.

Type	Primary use	Benefits
Borosilicate 3.3	Covering, evaporation, weighing	Autoclavable, low thermal-shock risk, durable
Quartz	High temperature, UV/photochemistry	Very low expansion, optical purity
PTFE	Highly corrosive media, HF	Chemical inertness, non-stick
Plastic (PS)	Single use, sensitive areas	Lightweight, economical, no washing
Soda-lime	Room-temperature covering, simple weighing	Lower cost, basic use

In which fields is a laboratory watch glass used?

• Life sciences: gentle solvent evaporation, drying precipitates, covering beakers to limit contamination during non-sterile culture work.
• Pharmaceutical quality control: stepwise weighings, drying residues, observing crystallisation.
• Chemistry and cosmetics: evaporation on a hotplate, splash protection during reactions, solubility testing.
• Environment and food testing: drying deposits after filtration, evaporation to determine dissolved solids.
• Teaching/R&D: demonstrations, routine handling, dust protection at the bench.
• Cleanroom/production: disposable versions to minimise particulate transfer where washing is not permitted.

Available types of watch glasses

Watch glass

Frequently asked questions about laboratory watch glasses

What diameter should I choose to cover a beaker?
Choose a diameter 20–30% larger than the beaker opening to cover it while still allowing vapour to escape.

Can I autoclave a watch glass?
Yes for borosilicate 3.3 and quartz. Avoid soda-lime glass and most plastics (e.g., PS), which are not designed for autoclaving.

Is it compatible with hydrofluoric acid (HF)?
No for all glasses (silica is attacked). Use a PTFE model, which is inert to HF and many corrosive agents.

Can it be heated directly on a hotplate?
With borosilicate/quartz, yes for small volumes. Increase temperature gradually and avoid thermal shock (dry, even surface).

Can a watch glass replace a weighing dish?
For straightforward weighing, yes. For high-accuracy analyses, use dedicated weighing dishes or calibrated, antistatic weighing boats.

How can I extend its service life?
Wash with a non-abrasive detergent, rinse with deionised water, and dry without impacts. Discard any item that is chipped.

Can it be used as a “sealed lid”?
It is not airtight: it acts as a splash guard and limits evaporation, but allows gases to pass. For a tight seal, use an appropriate closure.