By Geoff Browne, Senior Project Manager & Crating Specialist, Terry Dowd, Inc.
Art Packing and Transport: Cushioning Foam Selection and Application
When I began working in the field of fine art and artifact packing and crating in 1990, the information available regarding the correct application of packing foams, taking into consideration object fragility, weight, load bearing surface, and the likely drop height of a given package, existed almost entirely in hard copies in libraries or in corporate literature intended for industrial users, and was difficult to acquire even for those who knew exactly what they were looking for.
While most if not all of that same information can now be found online, the search can still be daunting due to the proliferation of vendor websites for packing materials which do not have the application information on their sites or links to it.
In addition, much of that information, even if found, represents only the data specific to a commercial product and / or is exclusively geared to packaging engineers and technicians who already understand its application. One of the few exceptions to this is the publications that arose from the Art in Transit Workshops organized by the Canadian Conservation Institute, The Smithsonian Institution, The National Gallery, and the Tate Gallery in 1991, and even those publications can overwhelm many employed in the field that are not as well grounded in the sciences as conservators.
When I was invited to give a presentation at the Campbell Center for the Packing and Crating Information Network of the American Association of Museums in September 2013 on the subject of my choice, the decision was easy, as I am almost daily presented with either packaging specifications or actual packaging examples which do not take correct foam loading principles into account.
The following paper is, I trust, a more fluid and self-explanatory version of my presentation for PACCIN in 2013, and something which I personally would have really appreciated when I started working in this profession.
Cushioning Foam Selection and Application
The fragility of objects, shock levels, and cushioning values are expressed in G forces or “G”s.
Fragility is expressed in terms of how many G forces an object can withstand before damage occurs;
Shock levels are expressed in terms of how many G forces are applied to an object upon impact with an immovable surface given a specific drop height –
- In common usage, the term “G forces” is most often applied to the acceleration of racing cars, high performance aircraft, spacecraft, etc., forces which however rapid and potentially dangerous to living things are nonetheless applied relatively gradually and are mild, typically in single digits, as compared to G forces measuring shock values, which represent the force applied to a free falling and constantly accelerating object at the instant it impacts an immovable surface such as a concrete floor, which if dropped from a height of only 12” can be 50 Gs.
- G-forces times the weight of the object at rest = the force applied to the overall object upon impact expressed in pounds.
- Dividing overall force by the area of the object actually making contact with the “floor” upon impact = “point loading” force in pounds per square inch.
If a 10 lb. object is dropped from a height of 12” onto a concrete floor, yielding a 50G shock, the overall pressure applied to the object upon impact is 500 lbs. If the object is a 10” cube that happens to land perfectly on one of its flat sides, the surface area of the object actually making contact with the floor upon impact is 100 sq. inches, and the locally applied pressure is only 5 pounds per square inch. If, however, the cube landed on a slightly rounded corner with a contact area of 1/10 square inch., the locally applied pressure would increase to 5,000 psi.
Cushioning values are expressed in terms of the extent to which a properly loaded material can limit the G forces experienced by an object dropped from a given height onto an unyielding surface.
Object fragility ratings:
(US Dept. of Defense MIL-HDBK-304c & Dow Ethafoam Guide)
|Extremely Fragile||15 – 25 Gs||Altimeters, hard drives, missile guidance systems, precision aligned test equipment, gyroscopes, inertial guidance systems|
|Very Delicate||25 – 40 Gs||Altimeters, digital electronics equipment (hard drives), medical diagnostic apparatus, X ray equipment, mechanically shock mounted instruments|
|Delicate||40 – 60 Gs||Computer display terminals and printers, electric typewriters, cash registers, aircraft accessories; Typical Paintings – 50 Gs; Glass Bottle – 60 Gs (Art in Transit)|
|Moderately delicate||60 – 85 Gs||Stereos and television receivers, floppy disc drives, aircraft Accessories|
|Moderately rugged||85 – 115 Gs||Major appliances and furniture, electromechanical equipment|
|Rugged||115 + Gs||Table saws, sewing machines, machine tools, aircraft structural parts such as landing gear, control surfaces, hydraulic equipment|
Shock levels experienced on various modes of transportation, sensors secured to vehicle:
(American Association of Railroads Intermodal Environment Study & MIL-HDBK-304c)
|45’ Non air-ride trailer, 1900 miles on highways, 1400 miles on urban streets||Vertical shock: 95% < 4 Gs;
Longitudinal shock: 95% < 1 G;
Lateral shock: 95% < 2 Gs;
|Non air-ride trailer, semi-loaded, composite||
Vertical shock: 99.5% < 4 Gs, 1 – 300 Hz
|Air ride suspension trailer, measured on floor||
Vertical shock: < 1 G peak, 2 – 500 Hz
|Jet Aircraft, vertical shock and vibration composite||
Vertical shock: 3 G peak, 1 – 2000 Hz
Shock levels for perspective, from crude testing:
|Rapping a pen (accelerometer) on a desk||20 Gs|
|Dropping a slab of wood 12” onto concrete||
Probable Drop Heights of packages by Weight:
(US Dept. of Defense MIL-HDBK-304c & Dow Ethafoam Guide)
|Package Size||Handling||Probable Drop Height|
|0 – 10 lbs.||1 person throwing||42 inches|
|10 – 20 lbs.||1 person carrying||36 inches|
|20 – 50 lbs.||1 person carrying||30 inches|
|50 – 100 lbs.||2 people carrying||24 inches|
|100 – 250 lbs.||light equipment||18 inches|
|250 + lbs.||heavy equipment||
Other Reference / Notes:
Incredible as it may seem when considering the very low shock levels experienced on air-ride trailers, consider the fact that ripe tomatoes are routinely successfully shipped to market in refrigerated air-ride trucks over thousands of miles, with tomatoes stacked atop each other in packaging rarely exceeding simple wooden slat crates – a 2G shock, effectively doubling the weight of a tomato, could easily rupture its skin.
Please note that the shock levels provided for vehicles alone were necessarily measured against the structure of the vehicle, as if the sensors were secured like cargo – this is crucial.
In American Society for Testing & Materials (ASTM) protocols for package testing, one of the test regimens is referred to as a “Loose Load” test, where the cargo is totally unsecured in the vehicle, as is commonly the case with express package carriers and some common carriers – in that case, individual G forces of shock could easily exceed the 50G level, and the cargo could also be subjected to repeated shocks, resulting in cumulative damage that isolated events might otherwise not cause.
Principal Cause of Damage Due to Shock:
If one is shipping via modes of transportation where cargo is secured, especially if the carrier vehicle is an air ride trailer, the rather inescapable conclusion one may derive from the foregoing tables is that carrier vehicles are not, in and of themselves, the major source of potential damage, but rather that the handling of the objects in transitional stages is the likely source of damage in any transport sequence.
Cushioning material selection and application:
Cushioning foams act as shock absorbers, returning to shape slowly after impact - as opposed to
Upholstery foams, which act like springs, bounce back quickly, and result in significant rebound shock and vibration.
Upholstery foams are additionally in most cases polyurethane compounded with Ether, whose solvent properties are of course hazardous to most materials, including the foam itself, making it prone to rapid breakdown.
Open celled foams (typically polyurethane, as is sound proofing foam) are inherently better at damping vibration than closed celled foams (typically polyethylene foams, which are also “harder”).
Both of the most common varieties of packing foams are available in military grades which are reputedly employ better chemical compounds and or use better controlled processes to lessen any chemical reaction with items to be packed. Unfortunately, in the case of Ethafoam, the blowing agent (which inflates the foam cells during manufacturing) used in the Military grade is proprietary…
Zote foams (Plastazote) are cross linked polyethylene, their cells being filled with Nitrogen while the polyethylene is subjected to strong vacuum.
Packaging Foam Comparison and Static Loading tables:
All figures shown are based on a 30” drop, considered the maximum likely drop in truck shipments.
Static load values shown are from manufacturers cushioning curves based on multiple impacts to allow for hard usage;
Static loading values shown will yield optimum protection / lowest possible G forces for that type and thickness of foam, as shown in right hand column.
|Foam Type||Thickness||Static Loading – object lbs. per sq.in. foam (psi)||Static Loading – as sq. in. foam required per object lbs.||G force
|Polyester Urethane||2”||.375 psi.||2.67 sq.in./lb.||37 Gs|
|Polyester Urethane||4”||.375 psi.||2.67 sq.in./lb.||14 Gs|
|Ethafoam 220||2”||.67 psi.||1.67 sq.in./lb.||46 Gs|
|Ethafoam 220||4”||.85 psi.||1.17 sq.in./lb.||24 Gs|
|Plastazote LD24||2”||~ .5 psi.||~ 2 sq.in./lb.||60 Gs|
|Ethafoam HS 45||2”||.75 psi.||1.33 sq.in./lb.||
|Ethafoam HS 600||2”||1.2 psi.||.833 sq.in./lb.||
|Ethafoam HS 900||2”||2.2 psi.||.454 sq.in./lb.||
To pack a modern painting in a simple rectilinear frame measuring 30 x 36 x 2”, weighing approximately 10 lbs., with a nominal fragility rating of 50Gs, for truck shipment:
Either 2” thick Polyester urethane foam or 2” thick Ethafoam 220, correctly loaded, would provide adequate protection, at 37 Gs and 46 Gs respectively.
Using 2” Polyester urethane foam, a total of 26.7 sq. inches of foam in contact with the frame would be required (2.67 sq. in. foam per pound x 10 lbs.);
Given the 2” depth / width of the frame, a theoretical single cushion would thus need to be 13.35” long x 2” wide – in actual practice, two cushions, one at each corner, would each need to be 6.68” long x 2” wide, typically with additional width extending to back and lid of crate to prevent buckling of foam at its edges or knifing between layers of foam – see Additional Considerations in Cushioning Material Usage.
Other cushioning materials frequently employed in tandem with foregoing foams:
100% Polyester, low melt heat fused vs. resin bonded – cushioning for extremely lightweight and / or irregular artifacts, relative inert, easily worked
large cell, low density polyethylene foam, good soft initial surface for heavier objects when applied over harder grades of Ethafoam
Cross linked polyethylene, good soft initial surface for heavier objects when applied over Ethafoam, but crushes and tears easily, should only be presented directly to object surfaces, not slid against
Additional Considerations in Cushioning Material Usage:
When an object requires rigid support so that components cannot move relative one another, or to support a work which is structurally weak due to its media, an inner case should be made to provide that rigid support, and the inner case is then cushioned by foam elements within the outer crate shell.
Small lightweight / low mass objects extremely sensitive to vibration, such as a pastels, can also benefit from the added mass of an inner case, as the overall resonant frequency is lowered. An inner case also provides greater and safer surface for application of cushioning foam.
Fit / Clearance
Ideal Clearance = 0, with / if Friction also = 0 - perhaps most nearly achieved with foam and object both wrapped in slippery material. An object should easily lower into place within foam cushioning elements under its own weight.
Excess clearance allows rebound, thereby adds to total shock and vibration. Excess tightness promotes abrasion of object on insertion / removal, or possible damage, and can interfere with cushioning (e.g. If side foam too tight, bottom foam cushioning diminished or negated).
Finger or hand width gaps to lift out or load objects must be provided, and should be in proportion to weight of object as object increases in size – allow access for 2 hands per 50 lbs. of object. Handling cutouts also provide positive identification of correct handling areas for future handlers.
Air traps occur most often with large flat works, trays, or inner cases with tightly fitted cushioning having few or small gaps between cushioning blocks, and thus little or no means for air movement around inner case or object. Vacuum or air pressure created by air traps diminishes or negates cushioning. To minimize, allow larger gaps between cushions, which may require use of foam with higher load bearing capacity; trays may have corners cut off to allow greater flow.
Foam placement in package / relative object form
Foam should be placed at points where the structure of the object can safely support its own mass under stress, e.g. at the corners of a frame or stretcher rather than in the middle where no bracing may exist, etc.
NOTE: When more than one grade of foam can be properly loaded and applied in the cushioning of an object, the foam requiring the greatest area will provide the most even support and likely the best cushioning value as well.
Toppling is a rotational drop, such as a painting crate falling over on its face or back. Toppling is equivalent to a flat drop from 2/3 the height of the crate. To best address this issue, the use of 4" foam on front and back faces is recommended both for its greater cushioning value, and as the additional depth of the crate will decrease the likelihood of toppling.
Travel beyond static positioning, layout
Foam applied in a single container should not only extend the entire depth of the face against which it bears, but also extend beyond that face by the thickness of the cushioning material on the adjacent sides - e.g. floor foam for a painting crate should generally extend from the full depth of the crate, so that if the adjacent side cushioning bottoms out under severe impact, the object does not drop off the floor cushioning.
Buckling is the result of high loading at or near the edge of foam cushioning. If the object has narrow load bearing surface, such as a painting with shallow frame profile, bottom foam should extend from front to back of crate.
If a foam element is proportionately taller than wide, buckling becomes likely – the use of foam elements at least one and one third times as wide as thick / tall is recommended.
If voids exist in corners of package / between floor and side foam elements, a corner drop may cause buckling due to angle of impact - additional foam should be added to the floor foam to fill corner voids.
Fatigue / Compressive Creep
Foam fatigue results from continuous object static load on foam and from repeated shock or severe environmental conditions, and may be offset by addition of ~ 10% additional load bearing foam area.
Platens (Load Spreaders)
Platens are rigid flat plates which increase the load bearing surface of an object to permit correct cushioning of heavy objects; in the case of hollow metal castings they also prevent excessive point loading & knifing of media through foam cushion; they are also useful in facilitating lateral transfer of an object into its container if they have a slippery surface.
Platens also allow better cushioning of lightweight objects while evenly distributing load.
Progressive cushioning may be achieved by the mixing of cushioning materials or densities of foam, such as by placing fiberfill pads atop Urethane foam, or Volara or Ethafoam 222 atop other grades of Ethafoam - care must be taken, particularly with fiberfill, to limit the thickness of the innermost / softest material to avoid rebound, as with excessive clearance.
Progressive cushioning may also be achieved via use of trapezoidal cushioning elements, which upon compression not only become denser upon compression, but also engage more surface area as they compress.
Usage of foam as insulation
While Urethane foams in particular have R values nearly equivalent to high density polystyrene such as Dow Blueboard, USG Pinkboard, etc., the usage of foam as insulation can easily result in overfoaming if the insulation layer / lining is also the cushioning layer and no consideration is given to correct loading.
Cavity packs / full encapsulation of objects in foam naturally creates insulated packaging.
As I was gathering reference material for my presentation for PACCIN in 2013 and this paper, I was reminded that far and away the greatest risk to any item in the shipping process is from handling, NOT from the mode of transportation itself, except in the case of a “loose load”, which explains how soft packed artwork can be transported on air-ride trucks IF properly secured and handled properly.
The one consideration which is not addressed in this paper is cost analysis, which tends to be a lesser consideration for institutions, whose mission stresses preservation. As a taxpayer, I was of course pleased to see that cost analysis is addressed in detail in the Department of Defense publication.
For those interested in further exploration of these topics, I strongly suggest that you acquire copies of “Art in Transit: Studies in the Transport of Paintings” ISBN 0-89468-163-X, the “Art in Transit Handbook” ISBN 0-89468-165-6, and the U.S. Dept. of Defense publication, MIL-HDBK-304c, which can be downloaded for free online - it is very thorough, over 100 pages long, and includes free software as well if desired.
Prepared by Geoff Browne
Senior Project Manager & Crating Shop Manager
Terry Dowd, LLC | Chicago