Specifying a display case

A display case does several jobs that pull in different directions. It keeps people and dust away from the object, it can hold a stable humidity, and, if built from the wrong material, it concentrates corrosive vapours around the object it was meant to protect. A specification that names only dimensions and glass thickness will produce a case that does the first job and fails the others.

Air exchange rate

This is the master specification. It describes how quickly the case swaps its interior air with the room, and it decides both how well the case can buffer humidity and how badly any internal emission accumulates.

A case at a low exchange rate can hold a humidity band for months on a modest quantity of conditioned sorbent. A leaky case cannot be held at any practical quantity, because it is continuously importing the room. The difference between a well-made and a poorly made case is an order of magnitude, and it comes from workmanship rather than from design.

Ask for a stated and, ideally, tested figure. A supplier who has never measured one has not been building sealed cases.

Materials

Every material inside a sealed case will be in contact with its atmosphere for the life of the case. Wood, wood-based boards, many adhesives and sealants, PVC and some fabrics emit organic acids or sulphur compounds that corrode metals and damage shell, calcareous material and some glass.

The established safe list is short and adequate: glass, powder-coated steel with a fully cured coating, anodised aluminium, stainless steel, polyester, polyethylene and polypropylene. Anything else going inside should have an Oddy test result, and that includes paints, fabrics, mount board and adhesives, not just the structure.

A specific caution about coatings: a powder coating that has not fully cured emits. So does a freshly painted deck. Cases should be built, cured and, if practical, aired before objects go in.

Glazing

Laminated glass is the usual choice, giving security and blocking a substantial share of ultraviolet through its interlayer. For anything relying on that, measure rather than assume, since the amount depends on the interlayer. UV filter film covers the retrofit option.

Anti-reflective coatings improve visibility considerably and cost accordingly. Low-iron glass removes the green cast that is visible against pale objects. Both are display decisions rather than conservation ones, but a case people can see into is a case that gets fewer fingerprints on the glass.

Access

The access panel is where most leakage occurs and where most handling risk sits. A door with a good gasket and even clamping pressure seals far better than a lift-off panel that relies on weight.

Consider what opening the case involves. If two people and a suction lifter are needed, the case will be opened less often, which is good for the microclimate and bad for the object nobody has inspected in three years. The compromise most institutions reach is a case that seals well and opens deliberately rather than easily.

Lighting

Lighting outside the case, directed through the glazing, avoids three problems at once: heat inside the enclosure, a cable penetration that compromises the seal, and the need to open the case for lamp maintenance.

Where internal lighting is unavoidable, it should be in a separate, ventilated compartment isolated from the display volume, with LED sources chosen for low heat and no UV.

Climate provision

If the case is doing climate work, the specification needs a place to put the sorbent and a way to service it: an accessible compartment with a perforated deck allowing air exchange with the display volume, sized for the quantity you calculated rather than for whatever fits.

It also needs somewhere for a logger, and ideally one that can be read without opening the case. Showcase microclimates covers the operation side.

What to write into the specification

• Target air exchange rate, and whether it will be tested on delivery
• Full material schedule with test results, including finishes, adhesives and mount materials
• Glazing type, UV performance and coating
• Access method, gasket type and expected opening frequency
• Lighting position, source type and maintenance access
• Sorbent compartment volume and servicing route
• Logger position and readout method
• Cure and airing period before objects are installed