You do not always need an expensive digital moisture meter to find out if your firewood is ready for the stove. In fact, one of the most reliable and entertaining ways to test kiln-dried alder logs involves nothing more than a little washing-up liquid and the breath in your lungs. It is called the bubble test, and it works remarkably well on alder.

The Science of Wood Pores

To understand why this test works, picture wood as a bundle of microscopic drinking straws. Trees stay alive by drawing water from the soil up to their leaves through hollow vascular channels called xylem vessels. In living or freshly felled wood, these channels are completely gorged with liquid water, sap, and plant sugars.

When firewood is properly kiln-dried, that moisture is evaporated out of the wood. The water boils away, leaving open, uninterrupted air channels running straight through the log from end to end. If a log is still wet or improperly seasoned, those channels remain clogged with water and sticky sap plugs.

This drying process is especially clean in alder, as explored in Kiln Dried Alder: Low Moisture Secrets for an Effortless Blaze. Because alder is diffuse-porous, its vascular channels are open, straight, and evenly distributed across the entire log cross-section.

How to Perform the Bubble Test Step by Step

Performing the test takes about thirty seconds. Here is how to do it at your hearth:

1. Select a fresh split: Take an average-sized split log of alder, roughly 20 to 25 centimetres in length.
2. Apply washing-up liquid: Rub a generous smear of standard liquid dish soap across the exposed grain on one end of the log. Make sure the soap covers the wood pores cleanly.
3. Blow through the log: Place your mouth firmly against the dry, opposite end of the log, forming a tight seal around the wood fibres with your lips. Blow hard through the wood.
4. Watch the soapy end: Look closely at the soapy surface on the far side.

If the alder is genuinely kiln-dried with low moisture, you will see frothy soap bubbles form and expand across the soapy end of the log within seconds. You are literally blowing air straight through the vascular network of the timber.

What the Results Mean

If air passes through easily enough to blow bubbles, it proves two crucial things:

• The internal xylem vessels are completely free of water and moisture barriers.
• The wood has cured thoroughly to its core, meeting the standard discussed in What Does Sub-20% Moisture Mean for Alder Combustion?.

Why Alder is the Champion of the Bubble Test

Try doing the bubble test with dense oak or hornbeam, and you will usually fail even if the wood is dry. Those timbers contain tyloses—balloon-like outgrowths that naturally plug their vessels during tree maturation. Alder, however, has wide open vessel paths. This makes it the perfect timber for testing, and it is the exact physical characteristic that allows the log to take a flame so fast once lit in your hearth.

While the bubble test is a brilliant party trick and a dependable practical check, commercial buyers should still ensure their wood carries formal third-party backing. Read Ready to Burn Certified Alder: What UK Buyers Need to Know to understand how official Woodsure certification guarantees stove performance.

Try It at Home Tonight

Next time a delivery of kiln-dried alder arrives at your door, grab a bottle of washing-up liquid and test a log for yourself. Watching soap bubbles blow out of solid hardwood is not just fascinating; it gives you total confidence that your firewood is bone dry and ready to produce an effortless, roaring fire.