Ultrasonic cleaning is remarkably gentle and remarkably powerful — and that combination means a few items shouldn’t go in the bath. The same cavitation that lifts grime from a watch bracelet can loosen a glued setting or worsen a hairline crack. Knowing what to keep out protects your valuables and your equipment.
Here’s a clear, practical list of what not to put in an ultrasonic cleaner, and why.
Soft, porous or heat-sensitive gemstones
The cavitation and warmth that suit hard stones can damage softer or porous ones. Keep these out of the ultrasonic bath:
- Opals, pearls and coral — porous and easily cracked, crazed or dulled.
- Emeralds — commonly oil- or resin-treated; ultrasonic cleaning strips the treatment and can worsen their natural inclusions.
- Turquoise, lapis lazuli, malachite, onyx and other porous/dyed stones — can absorb liquid, lose colour or crack.
- Tanzanite, topaz, amber, jet and moonstone — sensitive to vibration and temperature shock.
Hard, untreated stones such as diamonds, sapphires and rubies are generally fine — but if a piece is antique, fracture-filled or you’re unsure, clean it by hand.
Jewellery with vulnerable settings or finishes
- Glued or pressure-set stones — vibration can loosen adhesives and shake stones free.
- Plated jewellery (gold or rhodium plate) — repeated ultrasonic cleaning can wear thin plating.
- Antique, fragile or already-cracked pieces — existing weaknesses can propagate.
- Costume jewellery — mixed materials and glued components don’t tolerate it well.
Certain metals and finishes
- Aluminium and soft metals — strong alkaline solutions can etch or discolour them; if you must, use a material-safe neutral solution and test first.
- Painted, lacquered or coated parts — ultrasonics can lift the coating.
- Soft or reactive metals like raw brass can spot or tarnish in the wrong chemistry — match the right cleaning solution to the metal.
Watches and electronics (with caveats)
- Non-sealed watch movements and dials — water ingress can ruin them. Only fully sealed cases by a watchmaker who knows the rating.
- Items with batteries or live electronics — remove power sources first; uncoated boards may need a specialist flux-removal process rather than a general bath.
Don’t use the wrong liquid, either
- Flammable solvents (petrol, acetone, pure alcohol) must never be used in an ultrasonic tank — a serious fire and fume risk. Use purpose-made water-based concentrates.
- Household detergents foam, and foam kills cavitation — see our guide to ultrasonic cleaning solutions.
- Bleach corrodes stainless tanks and should be avoided.
A few good habits
- Use an insert basket so items never sit directly on the tank floor (direct contact can damage both the item and the transducer).
- Don’t overcrowd — parts need solution around them to clean.
- When in doubt, test on an inconspicuous area or clean by hand.
Still unsure?
If you’re not certain whether an item is ultrasonic-safe, it’s always better to ask first. Browse our ultrasonic cleaner range and cleaning solutions, see our guide to choosing a cleaner, or contact our New Zealand team for advice on your specific items.
For nearly 38 years, Tim Fastnedge has been a driving force behind Techspan’s expertise in ultrasonic cleaning equipment. His knowledge of ultrasonic cleaners is extensive, allowing him to recommend the most effective cleaning solution for any application.
Tim’s dedication extends beyond sales; he’s passionate about empowering customers with the right tools to achieve optimal cleaning results. So when it comes to Elma Ultrasonics New Zealand’s range of high-quality equipment, look no further than Tim’s expertise at Techspan.