Deposition, adhesion at temperature & gradient control
Five things that decide whether a metallized candle jar survives its first burn.
Vacuum metallizing deposits a metal film onto the outside of the glass, and masking or rotation during deposition produces a gradient that fades from dense to transparent. It is one of the most striking finishes available and one of the most demanding, because a burning candle heats the glass exactly where the film is thickest and most visible.
The film is deposited, not plated in a bath
Vacuum metallizing evaporates metal in a chamber and condenses it onto the glass as a film measured in nanometers, usually with a base coat below and a protective lacquer above. Understanding that it is a three-layer stack rather than a single coating explains most of its failure modes: any one of the three can be the weak layer.
Adhesion is tested with a candle, at the wax line
A metallized jar spends its life with a heat gradient across it, hottest at the surface of the melt pool. That line moves down the jar as the candle burns, so every part of the coated area is heated in turn. Testing a coated sample in an oven misses that traveling boundary, which is where crazing and lifting actually start.
A gradient is harder to control than full coverage
Fading from opaque to clear means the film thickness varies continuously, and thin regions are both more fragile and more sensitive to deposition variation. Batch-to-batch consistency in where the fade begins and how quickly it clears is the property that most often disappoints, and it is agreed with limit samples rather than a written description.
The coating stays outside the container
Metallizing is applied to the exterior only, so nothing coated ever contacts the wax. That keeps the question purely one of appearance and durability rather than of what might migrate into a fragranced product, and it is worth stating explicitly because customers frequently assume the reverse when they see a mirrored interior effect.
Handling marks show more than on any other finish
A polished metallic surface records fingerprints, scuffs and packing marks with unusual clarity, and a scratch through to the glass cannot be touched in. That drives the carton design, the interleaving and often a decision to ship in individual boxes, which is a cost that belongs in the finish decision rather than appearing later.
The gradient has to start above the highest wax line
Where the coating meets the heat of the melt pool it is at its most vulnerable, so the fade is planned to reach full coverage only above the level the wax ever reaches. Setting that boundary from the fill height rather than from the drawing is what keeps the finish intact through the burn.
