On a cloisonné vase the fine gold lines look like drawing. They are not. Each one is the top edge of a low copper wall bent by hand, fixed to the copper body, and only then filled with colour. The walls come first. The design is a consequence of where the walls were put, not the other way round.
The reason is that enamel is a powder which becomes a liquid. Packed into the cells as a damp paste and fired at around 800 degrees Celsius, it melts, levels itself, and would happily run into its neighbour. Two colours sharing an open surface would blend into a third. The strips of copper hold each colour inside its own small compartment, so a red petal stays red right up to the line that separates it from a blue ground. The word for the technique refers to these compartments, and the compartments are the whole idea.
A wall is also a thickness gauge
The height of the wire sets how thick the enamel layer can be. Fill past the top and the surplus has to be ground away later; fill well below it and the fired glass sits in a hollow with bare copper showing around it. Surface tension pulls the melt level within each cell and lets it climb slightly against the wire, which is why a finished piece shows a shallow curve if you look across it in raking light. The wire is a dam, a depth stop and a visible line at the same time.
Cell size is a structural decision too. A large pool of glass cools unevenly and shrinks enough to crack as it sets. Dividing the same area into many small cells lets each one take up its own contraction without tearing the surface apart, so the dense, busy patterns associated with the craft are not merely a taste. A glaze that is allowed to crack shows the other side of the same problem: there the crazing is accepted and made into a surface, here it is prevented by giving the glass nowhere to pull away.
The wire itself is thin enough to bend around a tight curve yet stiff enough to stand through filling and firing. It is usually fixed to the body before the first firing, so the first thing that happens to the object is that a flat metal surface acquires a relief map of shallow channels. Only after that does colour arrive, applied cell by cell with a small spatula rather than brushed across the whole piece.
Why it goes back into the furnace
A paste of powdered glass contains a great deal of air. Melted, it collapses into a much smaller volume, so one firing leaves every cell visibly sunken. The piece is refilled and fired again, and this is commonly repeated three or four times before the enamel comes up level with the wire. Each round is a fresh chance for a cell to be missed, a colour to be contaminated, or a wall to be knocked out of shape.
Every round is also a thermal test. The copper body and the glass must expand and contract at reasonably close rates; if they differ too much, the cooling glass pulls away from the metal or flakes at the edges. This is the same negotiation a bowl's foot ring makes with firing shrinkage, except that here two dissimilar materials are shrinking against one another rather than a single clay body managing its own. The copper is not just a support, it is part of the thermal calculation.
When the enamel is finally level, the surface is ground flat with progressively finer abrasives. This removes the glass that stood above the wires and exposes their tops, which are then gilded so they read as gold lines. Grinding is not a polishing afterthought: it is the step that decides whether the lines are continuous. Under-grind and the lines appear in broken stretches; over-grind and the cells become shallow dishes.
What the wire is not
It is easy to mistake the wires for an outline added at the end to sharpen a painted image. But an outline can be any height, take any route, and be changed while the work is in progress; a wall cannot. Once the walls are fixed, the number of colours, the size of each field, and the thickness of the glass are all settled. Even the choice of a many-petalled flower over a broad plain ground is partly a decision about how many cells will be needed to keep a large sheet of glass from cracking.
Other coatings solve the same problem differently. Lacquer cures into a film that stays where it is brushed, so it needs no walls at all, but it demands humidity rather than heat to harden. Enamel needs heat, and heat makes it flow. The copper strip is the price of using a material that becomes liquid before it becomes a surface.
One look settles it. Hold a piece sideways to the light and find a cell where the glass sits a hair below the wire on either side. That tiny step is the record of every firing, every refill, and every pass of the grinding stone. The gold line is not describing the colour. It is what made that colour possible in the first place.