Reading an old bottle by its fill.

Ullage is the gap between the wine and the cork. On an old bottle it is the first thing to check. Wine fill levels record how a bottle has lived: how warm it was kept, how dry the air around it was and how well its cork held. The label tells you what the wine was. The fill tells you how much of that may be left.

Some ullage is normal and a little is inevitable. The gap grows slowly over decades, so a thirty-year-old Bordeaux with its wine at the base of the neck has lived well. What matters is the level set against the age. The same fill can be unremarkable on one bottle and a warning on another, which is why every level below comes with a rough guide to age.

What ullage is and why it grows

At bottling, the wine sits high in the neck with a small pocket of air below the cork. Over the years a little wine evaporates through the cork and the gap slowly widens. In cool, humid, steady storage the change takes decades to show.

Two things speed it up. Heat makes the wine expand and press against the cork. Every warm spell followed by a cool one works the seal a little looser. Dry air dries the cork, which shrinks and seals less well. The same wine can therefore show very different fills after thirty years depending on where it lay. In a climate like Hong Kong's, that difference is one of the best arguments for keeping wine in professional storage.

A larger gap usually means the cork has let more through in both directions: wine out and air in. So low fills and tired, oxidised wine often go together. Natural corks also vary from one to the next, so two bottles from the same case can sit at different levels; the guide to corks and other closures explains how they behave over time.

Wine fill levels on a Bordeaux bottle

Bordeaux bottles have a pronounced shoulder, so fill is described by where the wine sits against the neck and shoulder. The scale runs from top to bottom like this, with a rough guide to what is normal:

• Into neck. As filled. Normal for a young wine.
• Base of neck. Excellent at any age and normal for a wine of about ten years or more.
• Top shoulder. Normal at about twenty years or more.
• Upper shoulder. Acceptable at about thirty years or more.
• Mid shoulder. A real risk at any age.
• Low shoulder. Rarely worth the gamble.

Read it as level against age. Top shoulder is unremarkable on a bottle of twenty-five years and worth a question on one of twelve. Upper shoulder is acceptable on a forty-year-old bottle, but on a fifteen-year-old one it says something went wrong. Below upper shoulder, age stops being an excuse. At mid shoulder the cork has let a great deal through; some of these bottles are still good, but you cannot tell which until you pull the cork. Low shoulder is a bet on a rare survivor and the odds are poor.

A fill level means nothing until you set it against the age of the bottle.

Auction houses publish their own versions of this scale. Some add steps such as very top shoulder and the age bands differ from house to house, but the logic is the same. When buying at auction, ask for the level of every bottle in a lot rather than a general remark that levels are good for their age.

Burgundy bottles: fill in centimetres

Burgundy bottles slope gently from neck to body, with no shoulder to read against. Their fill is given in centimetres instead, measured from the base of the cork to the surface of the wine. Auction houses use the same convention for German bottles and other sloping shapes.

The logic does not change. A young wine should show very little gap and an older bottle may show more. As a rough guide, anything much beyond 4 to 5 cm deserves caution, although some very old bottles sit lower and still drink well. Ask for a number rather than a word and if the seller cannot give one, assume the answer is not flattering.

What to check alongside the fill

The fill is one witness. Four others are worth hearing before you believe it.

• Seepage. Dried wine on the glass around or under the capsule means wine has pushed past the cork, most often because of heat.
• The capsule. Sticky, stained or stuck fast to the neck points the same way. A capsule that still turns freely is usually a good sign.
• A raised cork. A cork pushed up above the lip of the bottle, often visible as a domed capsule, is a classic sign of a bottle that got too warm.
• Colour. Hold the bottle to the light or shine a torch through it from behind. A red or white much browner than its age would explain suggests oxidation, whatever the fill says.

One bad witness outweighs several good ones. The same checks sit within the wider list in how to check a bottle's provenance.

Why a low fill in a young wine is a red flag

Ullage from age is slow. If a wine of five or ten years already sits at top shoulder or below, something other than time has been at work: usually heat, sometimes a faulty cork, occasionally years standing upright in dry air. The missing wine is the lesser problem. A wine that got hot enough to push past its cork has usually been cooked as well and nothing you do afterwards brings it back.

Compare the bottles within a case. One bottle much lower than its neighbours probably had a poor cork, which is bad luck confined to that bottle. A whole case sitting low shared an experience, usually a hot one. Every bottle in it carries the risk.

Recorked bottles

Some estates have recorked old bottles for their owners. Done properly, each bottle is inspected, topped up if needed, fitted with a new cork and capsule and documented. A recorked bottle breaks the link between fill and history, because its level reflects the day it was topped up. Ask four things.

1. Who did the work: the estate itself or someone else?
2. When was it done and what documentation came with it?
3. What was the wine topped up with?
4. Was the wine assessed before it was resealed?

Estate recorking with records is reassuring. Recorking by an unknown hand puts a question mark over everything in the bottle.

How to photograph and describe a fill

Whether you are buying or selling, the same method gives a fair picture.

• Stand the bottle upright so the air gap sits at the top, then photograph it side on, at eye level with the shoulder, against a plain light background.
• For dark glass, shine a torch through the bottle from behind.
• For Burgundy-shaped bottles, hold a ruler against the neck with zero at the base of the cork.
• Photograph every bottle rather than one from the case, because fills vary from bottle to bottle.
• Add both labels and the capsule, seen from the top and the side.
• Describe each level in the standard terms or in centimetres, never as "good for age".

Treat the fill as the opening question. A good fill earns the bottle a closer look; a poor one needs an explanation and a lower price. Bottles you already own at mid shoulder or below will not improve, so move them to the front of the queue and open them with the care set out in opening an old bottle. If you have inherited or bought a cellar and want a view on what to open first, a cellar review covers it.