Glossary
Plain-language definitions of the wood, chemistry and aging terms used around the site. New here? Start with stave, char and toast.
A
- ABV (alcohol by volume)
- The percentage of a spirit that is pure alcohol. In the US, proof is roughly double the ABV — 50% ABV = 100 proof.
- Accent wood
- In a blend, a wood used in a smaller share (≤ ~30%) to add a targeted note — cherry for fruit, maple for sweetness — on top of the base wood.
- Alligator char
- The heaviest char level (Char #4): the surface cracks into shiny squares resembling alligator hide, from a ~55–60 second burn.
B
- Base wood
- The dominant wood in a blend (~60–80% of the surface area), usually oak, that provides the structural backbone — vanilla, caramel, spice — that accent woods sit on.
C
- Cask
- Another word for a barrel used to age spirits. A “cask finish” reuses a barrel (or wood) that previously held wine, sherry, rum, etc.
- Cavitation
- The rapid forming and collapsing of tiny bubbles in a liquid under ultrasound — the mechanism that speeds ultrasonic extraction.
- Cellulose
- The main structural fiber of wood. It’s more heat-stable than hemicellulose and lignin, so it contributes relatively little flavor.
- Char / Char level
- Burning the wood’s surface to a carbon layer, graded #1 (light, ~15s) to #4 (heavy, ~55–60s). Char adds color, caramelization and smoke, and filters out harsh compounds. Distinct from toasting.
- Congeners
- The flavor and aroma compounds in a spirit other than water and ethanol — many of them drawn from the wood.
- Cooperage
- The craft (or workshop) of making and charring barrels. A barrel-maker is a “cooper.”
- Coumarin
- A sweet, tonka/cinnamon-scented compound abundant in amburana wood. It’s regulated in food and drink because it can harm the liver in large, chronic amounts — so amburana is used only as a short finish.
- Cube
- A small block of oak (commonly ~1″×1″×1.5″, about 8 sq in of surface) used to age spirits. More surface area per piece than a stave, so it extracts faster.
D
- Distillation
- Concentrating alcohol by heating a fermented liquid and condensing the vapor. Note: aging/finishing an already-purchased spirit is not distilling — and home distilling is legally restricted in many places.
- Dominoes
- Short, flat segments of stave (like large dominoes) — a common, easy-to-handle home-aging format. See Stave.
E
- Ellagitannins
- A group of tannins in oak and chestnut that add structure, astringency and a drying grip. French oak is especially high in them.
- Entry proof
- The proof at which a spirit goes onto the wood. Around 110–125 proof pulls the most wood spice and tannin; lower proof favors sugars, vanilla and color.
- Esters / Esterification
- Esters are fruity, floral aroma compounds; esterification is the slow reaction that forms them from acids and alcohol during maturation — part of what makes an aged spirit taste mellow and complex.
- Eugenol
- A clove- and spice-scented compound released from oak lignin, driven up by toasting.
- Extraction
- Pulling flavor, color and compounds out of the wood and into the spirit. This is the part you can speed up a lot at home. Contrast with maturation.
F
- Fill proof
- The proof of the spirit when you put it in the vessel with the wood. See Entry proof and Proof.
- Finish / Cask finish
- A second aging step that layers in extra character — from a different wood, or from a barrel/wood that held wine, sherry or rum. See Cask.
- Furfural / HMF
- Sweet, caramel-and-toast-scented compounds formed when the wood’s sugars (hemicellulose) break down under heat.
G
- Grain (wood grain)
- The direction and pattern of the wood’s fibers. Long “face” grain extracts more slowly and evenly than cut “end” grain.
- Guaiacol
- A smoky, phenolic compound produced by heavier char — the source of campfire and medicinal notes.
H
- Heartwood
- The dense, dead inner wood of a tree — the only part you should use for aging. Bark, sap and green sapwood can carry off-flavors or toxins.
- Heat cycling / Temperature cycling
- Deliberately swinging the storage temperature warm and cool to speed maturation, mimicking a barrel warehouse through the seasons.
- Hemicellulose
- A wood component whose sugars break down under heat into caramel and toast compounds (furfural, HMF, maltol).
L
- Lactones (oak / whiskey lactones)
- Coconut- and fresh-wood-scented compounds naturally present in oak, best preserved by a lighter toast. Heavy char burns them away.
- Lignin
- The wood polymer that, when heated, yields vanilla and spice compounds — vanillin, syringaldehyde, eugenol and guaiacol.
M
- Maillard reaction
- The browning reaction between sugars and amino acids under heat that creates toasty, caramel and roasted flavors — the same reaction that browns toast and seared meat.
- Maltol
- A warm, caramel/cotton-candy-scented compound formed by a long, moderate toast; behind butterscotch notes.
- Maturation
- The slow oxidation and ester chemistry that mellows a spirit and builds complexity over time. Hard to fake at home. Contrast with extraction.
- Medullary rays
- Ribbon-like markings that run across oak’s grain — a visual cue that a wood really is oak.
N
- Neutral spirit
- A high-proof, essentially flavorless spirit (like vodka). Used here to soak and sanitize wood before aging.
- New make (new-make spirit)
- Freshly distilled, unaged spirit — sometimes called “white dog.” Raw and harsh until wood and time work on it.
O
- Oxidation
- The slow reaction of a spirit with oxygen during aging; it softens harsh notes and is part of maturation. A little helps; too much flattens the spirit toward cardboard.
P
- Phenolics / Phenols
- A broad family of wood compounds — vanillin, guaiacol, eugenol, tannins — responsible for much of oak’s flavor and color.
- Proof
- A measure of alcohol strength. In the US it’s twice the ABV — 100 proof = 50% ABV.
- Pyrolysis
- Breaking wood down with heat (toasting and charring). It’s the chemistry that creates most oak flavor — as opposed to a cask finish, which adds absorbed liquid residue instead.
R
- Rancio
- A rich, nutty, slightly funky “aged” character prized in long-aged sherry, brandy and rum.
- Rickhouse (rackhouse)
- The warehouse where barrels age. Its seasonal temperature swings drive natural heat cycling.
S
- Seasoning (of wood)
- Air-drying wood — months to years — before use, to shed harsh green tannins and mellow it. Cooperages season oak 9–24+ months.
- Solera
- A traditional fractional-blending system (used for sherry) where older and younger spirit are gradually blended over time.
- Sonic aging
- A marketing term for aging spirits by vibrating the barrels with loud, bass-heavy music (used by Conviction, Copper & Kings and Metallica’s Blackened). It’s audible sound, not ultrasound — a different thing from the ultrasonic extraction on the Accelerated Methods page.
- Sonication
- Applying ultrasound to a liquid to speed extraction. See Cavitation and Ultrasound.
- Spiral (infusion spiral)
- An oak stick cut into a spiral for lots of surface area; it drops into a bottle to flavor spirit quickly.
- Stave
- A long, flat plank of wood — originally one of the curved sides of a barrel. Cut-down staves (and “dominoes”) are a common home-aging format; they extract more slowly and evenly than cubes because most of the surface is long-grain.
- Surface-area-to-volume ratio (sq in/gal)
- How much wood surface touches each gallon of spirit. This site defaults to 40 sq in/gal; higher = faster and bolder, lower = subtler and slower.
- Syringaldehyde
- A vanilla-and-spice compound from oak lignin, more prominent in French oak.
T
- Tannins
- Astringent, drying compounds from wood that add structure and grip. A little gives backbone; too much tastes bitter. See Ellagitannins.
- Toast / Toasting
- Heating the wood’s surface (without flame) to develop vanilla, caramel and spice before any charring. Distinct from char.
- Tyloses
- Natural plugs that block the pores of white oak, making it watertight — the reason white oak, not red oak, is used for barrels. Basis for the white-vs-red-oak blow test.
U
- Ultrasound
- High-frequency sound used to speed extraction via cavitation. Real for pulling flavor out faster, but not a substitute for maturation. Note: “sonic aging” by some brands is loud music, not ultrasound.
V
- Vanillin
- The compound responsible for vanilla flavor, released from oak lignin — it peaks around a medium toast.
55 terms. Missing one you hit elsewhere on the site? It’s worth adding.