Glass bottles caps and autoclave cycles

A media bottle survives steam because of headspace, a cap that is allowed to vent, and glass that was made for the temperature step. This selection guide names the criteria to write down before you buy bottles and caps for autoclave work. It does not replace the autoclave manufacturer's cycle or your institutional sterilisation procedure. What steam does not accomplish is the subject of autoclaves and what sterilisation does not do. Where the bottle sits among other glass is covered in choosing laboratory glassware.

Criteria a specification should name

Name the glass. Laboratory media bottles that will see repeated steam cycles should be borosilicate, typically borosilicate 3.3, the low-expansion family in ASTM E438. Soda-lime bottles and recycled food jars are a poor thermal-shock match. They may hold a room-temperature reagent the glass tolerates. They should not be slipped into the autoclave basket because the thread looked similar.

Name the thread. GL45 is the common laboratory class: caps, pouring rings and many vented closures are made to it. A smaller bottle may be GL32 or another class. The number is the mechanical match. It is not a licence to tighten through resistance. If the cap does not run down by hand, the threads disagree.

Name the cap material and whether it is a plain closed cap, a cap with a liner, or a cap the manufacturer rates for venting during a closed autoclave cycle. Polypropylene caps are common and often autoclave-tolerant when the maker says so. A liner that suits aqueous media may fail in solvent. State the chemical the bottle will actually hold.

Name the volume and the fill rule. Choose a bottle larger than the liquid volume so you can leave headspace. Do not fill to the brim. Liquid expands as it heats, and a full bottle drives liquid into the cap and stresses the glass. A practical habit is to stay well below the shoulder for anything that will be steamed. The exact maximum is the bottle maker's instruction.

Name the cycle only as far as your autoclave procedure already defines it. This page will not invent a temperature, a pressure or a hold time. Those belong to the machine, the load and the validation your institution uses.

How the cap and the steam interact

An ordinary screw cap closed hard seals the headspace. During heat-up, air and vapour have nowhere to go. Pressure in the bottle rises faster than the chamber, liquid pushes at the thread, and cooling then pulls a vacuum that can distort a liner or suck airborne contaminants back in when the cap is finally cracked. Loosening the cap a quarter turn, or whatever your procedure specifies, lets the bottle track the chamber.

A vented cap is the exception, and only when the manufacturer describes it as suitable for autoclaving closed. The vent has to stay unblocked. Foil wrapped tightly over a loose cap can cancel the looseness. A pour ring left inverted, or a cap from a different thread, can do the same.

After the cycle, the bottle is a hot object that looks like a cold one. Thermal shock is the complement of a successful sterilisation: a sound cycle, then a crack on the bench because the base met a wet cold surface. Let the load cool. Tighten caps only once the liquid is near the temperature at which you will store it, or you trap a vacuum or a pressure the cap was not asked to hold overnight.

Branch when a load comes out wrong

Liquid in the cap and a wet basket usually means the bottle was too full, the cap was tight, or both. Reduce the fill and loosen the cap on the next cycle. Do not respond by forcing the cap harder.

A cracked shoulder or a star in the base means that bottle is finished. Do not autoclave it again "to be sure". New cracks after an otherwise normal cycle are a reason to look at shock on unloading, at chips that were already there, and at whether soda-lime slipped into the set.

A cap that no longer seals at room temperature may have a warped liner or a tired thread. Replace the cap. Do not wind tape around a GL45 neck and call it a seal for sterile storage. If the goal is only a clean bench reagent and not a sterile claim, say that in the label. The difference is explained by the cycle's limits, not by how carefully the tape was torn.

Volumetric flasks are the wrong bottle. Their stoppers and their calibration suffer in repeated careless heating, and a storage duty belongs in a media bottle. If you need a declared volume, make it up in volumetric glass, then transfer. NIST calibration practice is the background for that separation.

Caps wear out before the glass does

A borosilicate bottle often outlasts its cap. A liner cut by solvent, or a pour ring left on for a cycle the ring was not marked for, fails while the glass still looks new. Count caps as their own line in a specification. Repeat the thread, commonly GL45, on that line, and name the liner: a fluoropolymer face for broader chemical duty, or an aqueous-media liner if that is the truth. A bottle that arrives with the wrong cap is not yet a bottle you can put into steam. Replace the cap rather than winding a film around the neck and hoping the thread still means what it did.

Safety

A sealed bottle in an autoclave can fail violently. The rule is institutional and mechanical: follow the machine procedure, use bottles rated for the cycle, leave headspace, and loosen ordinary caps. Hot liquid in a cracked bottle is a burn and a spill. Biological media are a biosafety matter under the WHO laboratory biosafety manual and the CDC BMBL. This page is not a clinical sterilisation instruction and does not validate your autoclave.

Power, queues, and wet benches

Shared autoclaves in a busy building get opened onto crowded benches. Give bottles a heat-proof place to cool before they are packed against cold metal or rinsed. If a power cut stops a cycle, follow the autoclave's recovery instruction. Do not crack the door because the clock has run out. Inspect every bottle for new damage before the medium is trusted. Humidity does not replace headspace. A bottle filled to the neck because "it will be used today" is still a full bottle if someone else autoclaves it.

What to send

Glass type, nominal capacity, thread class, cap style including whether a vented cap is required, the chemical family the liner must tolerate, and that the duty includes autoclave cycles. State the number of bottles and caps separately, because caps wear out faster than glass. Use the laboratory glassware catalogue and the quote request. Plasticware alternatives, when a resin bottle is actually appropriate, belong in the laboratory plasticware catalogue and should not be substituted silently.