Highly or very highly vulnerable objects are defined by Stefan Michalski in "Quantified Risk Reduction in the Humidity Dilemma" (APT Bulletin, Vol. 27, Nº 3, , pp. 25–29). These definitions of object "vulnerability" are based specifically on mechanical damage. Please note that this classification has been applied mainly to wooden objects - so far. This information is intended as an indication of likely damage to certain materials and is included to exemplify the problems which may be encountered under different climatic regimes. It is not intended to be an exhaustive list of all possible vulnerabilities.
Classifying the Mechanical Vulnerability of Objects to Fluctuations in Temperature and RH
Objects can vary greatly in their vulnerabilities to fluctuations in temperature and RH. Stefan Michalski has defined four categories of vulnerability: very high, high, medium, and low ("Quantified Risk Reduction in the Humidity Dilemma", APT Bulletin, Vol. 27, Nº 3, , pp. 25–29; "Climate Control Priorities and Solutions for Collections in Historic Buildings", Historic Preservation Forum, Vol. 12, Nº 4, Summer , pp. 8–14). By way of example, a wooden object with aged glue, varnish, lacquer, gesso, or oil paint bridging a joint in which the grains of the wood components are at right angles to each other would be considered "very high" vulnerability. However, a single piece of wood with a paint or varnish layer would be considered a "medium" vulnerability object provided the wood had no cross-bars or restraints to its movement. Numerous examples of classified wooden artifacts can be found in the Vulnerabilities of Wooden Artifacts table.
The ASHRAE chapter uses these vulnerability classifications to describe the risk to collections from the various classes of control.
Vulnerabilities of Wooden Artifacts
This is adapted from the work of Stefan Michalski ("Quantified Risk Reduction in the Humidity Dilemma", APT Bulletin, Vol. 27, Nº 3, , pp. 25–29.
Very High Vulnerability
- ±5% RH: gradual fatigue fracture
- ±10% RH: fracture possible each cycle
- ±20% RH: fracture definite first cycle
- This class of wooden artifact breaks the rules of cautious woodworking, or else the fracture of these coatings has never been considered disfiguring, e.g. painted doors.
- Aged glue, lacquer, varnish, gesso, or oil paint that bridges joints where wood grains meet at right angles (lap joints, mortise and tenon joints, etc.; also, any knots in wood components).
- Aged glue, lacquer, varnish, gesso, or oil paint that bridges a crack formed by a check, knot, side-by-side butt joint, or mitre joint.
- Inlays of metal, horn, ivory, or shell (but not wood; marquetry is considered below as medium vulnerability, since it is much tougher and more resilient). The longer the inlay runs across the grain, the more vulnerable the piece.
- ±5% RH: zero fatigue fracture
- ±10% RH: gradual fatigue fracture, or plastic deformation
- ±20% RH: fracture possible each cycle
- ±40% RH: fracture definite first cycle
Much of this type of fracture has already occurred in old artifacts. Only artifacts from less fluctuating or higher annual average conditions, or those recently re-attached by inflexible treatments, will fall in this category.
- Veneer over corner joints where wood grains meet at right angles (lap joints, mortise and tenon joints, etc.; also, any knots in wood base components).
- Lacquer, oil paints, or gilding over single knot-free wood components, or over joints that use feathered inserts, fabric, tissue, etc. that are still sound. When new and fairly flexible, these layers may drop to only medium vulnerability.
- Fretwork, applied ornaments (especially if the wood grain follows the notch), and assemblies with metal bands, bolts, nails, or screws that restrain the wood unevenly.
- Checked timber or sculpture with a hard new fill.
- Cracked panel or panel painting in a rebate or cradle that jams.
- Large pieces of recently seasoned wood such as folk art, also all knots and uneven grain in wood must be considered prestressed in this way.
- New plywood, newly steam-formed wood held by other components.
- Panels near intermittently damp walls, floorboards over damp crawlspace, and glued veneer or joints where the glue bond has softened and readhered at the expanded component position.
- ±10% RH: zero fatigue fracture
- ±20% RH: gradual fatigue fracture, or plastic deformation
- ±40% RH: fracture possible each cycle
- Most wood joinery, veneers, and marquetry over single, clear pieces of wood at crossed grain; any prestressed pieces from above that have stress-relaxed for more than a decade.
- Any wood with little or no coating, subjected to an RH fluctuation shorter than its response time. This leads to warping or surface checking, e.g. backs of picture frames; on exposed end grain it results in end-checks, e.g. tenons, dovetails, feet of furniture, overhangs in carved totem poles.
±40% RH: possible accumulation of fatigue fracture or plastic deformation if the freedom to move, or the coatings, or the slowness of the fluctuation are less than perfect.
- Aside from possible cracks in any varnish, and given either a slow enough change in humidity or good moisture barrier coatings, then:
- already loose joinery
- floating panels
- loose tabletops
- tongue-and-groove or lapped planking nailed or bolted at one point only, e.g. wainscotting
- boxes on farm machinery (unless jammed due to painting or warping)
- hollowed-out totem poles
- hollowed-out sculpture
- most single-component tool handles
- veneer on wood with parallel grain
- joined wood components with parallel grain
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