Cork stopper, for Lab Use

Quick Answer: A cork stopper for lab use is a tapered plug cut from natural cork, the bark of the cork oak. It seals test tubes, flasks and bottles, and can be bored to take glass tubing or a thermometer.

Natural Cork and Where It Suits

Natural cork is made of tiny, air-filled cells with walls containing suberin, a waxy substance that makes cork light, springy and largely resistant to water. When the stopper is squeezed into a neck, the cells compress and push back, forming a seal.

The choice between cork and rubber is a chemical one. Cork tolerates organic solvents such as ether, chloroform and hydrocarbons that would swell and soften rubber, so traditional organic set-ups often used corks. But cork is attacked by strong acids, oxidising agents and halogens such as bromine, and alkalis darken it. For aqueous acids and alkalis a rubber stopper, or a plastic stopper where compatible, is better. Cork is also somewhat porous and can shed crumbs, so it is not suited to vacuum or pressure work, and some laboratories dip corks in molten paraffin wax to reduce porosity.

Size is chosen by taper. Roughly a third to a half of the cork should enter the neck. Rolling the cork under light pressure in a cork press, or on the bench under a flat board, softens it before insertion. For stocking several sizes at once, see the cork stopper pack.

Care & Handling

• Bore holes with a sharp cork borer slightly smaller than the tube, twisting from the narrow end and wetting the borer with water.
• Soften the cork by rolling before boring or fitting, to reduce cracking.
• Keep corks away from nitric acid, bromine and other oxidising or halogen chemicals.
• Store dry in a closed bag; damp cork can grow mould and very dry heat can shrink it.

Specifications

Applications

• Stoppering flasks and tubes holding organic solvents that would swell rubber
• Boring to take glass tubing, thermometers or condensers in traditional apparatus
• Closing sample tubes and bottles for dry solids
• Temporary closures in school practicals and demonstrations

Why Choose LabEquip

Organic chemistry labs and schools that need solvent-resistant closures still buy natural corks alongside rubber stoppers. LabEquip lists this cork in the General Lab Products range; send the neck sizes you need through the contact page.

Frequently Asked Questions

When should I choose cork instead of rubber?

Choose cork when the contents include organic solvents such as ether, chloroform or hydrocarbons, which swell rubber. Choose rubber for aqueous acids and alkalis, which attack cork.

Why does cork crumble when I bore it?

The borer may be blunt, the cork too dry, or the hole too close to the edge. Soften the cork by rolling it, use a sharp borer and twist it steadily.

Is a cork airtight?

It seals well for most bench work, but it is slightly porous, so it is not used for vacuum or pressure. Dipping corks in molten paraffin wax reduces porosity.

How do I pick the right size?

The narrow end should enter the neck easily and roughly a third to a half of the cork’s length should sit inside when fitted.

Can cork stoppers be heated?

Cork chars and shrinks with strong heat. Keep it away from flames and use it only where the vessel neck stays reasonably cool.

Can cork be sterilised?

Cork can be steamed or boiled, but it may soften, shrink or crack. Check the item’s rating before autoclaving and consider rubber or plastic closures for routine sterile work.

Last Updated: September 2026