Crating one of a kind high value items can be a science… when fabrication is in the hands of an expert. The following is an excerpt from one of many seminars performed by Geoff Browne at Terry Dowd, Inc.; this one pertaining to the cushioning of crates and the correct usage thereof:
Typical packing foams for the cushioning of crates are Ethafoam and Urethane. In both cases, the foam acts as a shock absorber vs. a spring (like upholstery foam) – if indented, it slowly comes back up to full thickness rather than immediately bouncing back. Ethafoam (polyethylene) is closed celled, with a high load carrying capacity, high durability, and is virtually chemically inert – for this reason it is often preferred for storage use, mount making, etc. This foam is typically white, and may be used commercially on high end heavy electronics, molded or cut to shape. Urethane (ester polyurethane or polyester urethane) foam is open celled, with a softer feel, lower load bearing capacity, and due to its physical structure, better at absorbing vibration – this type of foam, typically dark gray, is typically used in camera cases, and in “egg crate” configuration, used as sound insulation. Upholstery foam (polyether urethane), which is distinguished by, and undesirable for its springiness, is also undesirable for its chemistry – ether is a powerful solvent as well as an anesthetic, and can easily be identified by odor. Also as a result of its chemistry, it breaks down rapidly, becoming rigid and crumbly. Styrofoam, commonly used in commercial packing, does not provide the cushioning necessary for artwork, particularly in long haul situations or traveling exhibitions, as it is prone to permanently crushing following a single impact. Styrofoam should only be used as thermal insulation, and only in its high density form – Dow Blueboard, USG pink board, etc.
It has been determined that damage occurs to paintings and objects like green pottery when they receive a shock in excess of 50 Gs – to put that in perspective, rapping a pen on a desk results in a 20 G shock. Statistically, the smaller an object, the greater shock it is likely to receive in a shipping environment, as it can be easily dropped when hand carried from a height of about 30″, whereas a heavy object, which is likely to be moved by mechanical means is thus subject to much lower shock levels from a likely drop height of not more than 6″. Research done in the 1990s determined that cushioning for artwork should adequately cushion an object dropped from 30″. In order to adequately protect a typical object (painting, green pottery) a minimum thickness of 2″ of packing foam is required – one can think of this as braking distance – the greater the distance / thickness of foam, the gentler the cushioning – in the case of hyperfragile artwork such as pastels, 4″ foam may be desirable.
For typical 2.2 lb. density Ethafoam (Ethafoam 220), the weight of the object vs. the area of the foam should not exceed 1 lb. per square inch. For typical 2 lb. density ester polyurethane packing foam, the weight of the object vs. the area of foam should not exceed 1/2 lb. per square inch. It is frequently possible to use either type of foam for cushioning, in which case use of softer urethane foam, which would necessarily be applied over a larger area of the object, will provide more even support. If an object at rest compresses the foam it is placed on, an insufficient amount of foam is being used – the static load bearing capacity of foam can be as much as 10 times its dynamic load cushioning capacity – if foam compresses under object load at rest, it may already be severely compromised.
Pastel Crating & Illustration of Progressive Cushioning
Inner case containing pastel is suspended in trapezoidal foam elements in crate. By adding mass, the inner case lowers the resonant frequency of the object and thus the possibility of loss due to vibration. (Outer crate construction not shown)
View into open crate, inner case lid removed Tapered foam cushioning elements in outer crate provide progressive cushioning. As the foam is compressed, more foam surface is brought into action.
© 2002 Terry Dowd, Inc.