In mountmaking, the safety of the object always comes first and that applies when selecting your mount material as well. Some materials are already known to be safe and non-reactive, (such as brass, some stainless steels, acrylic plastic, etc.), some are known to be unsafe and shouldn’t be used (wool felt, rubber, etc.) while all other materials should be tested to determine if safe and non-reactive to use.
For more information on materials testing, see the AIC Conservation Materials wiki site
Your choice of materials will be determined by two other factors as well; the strength of the material (i.e. it’s ability to hold and support the object securely in position) and your ability to fabricate it into the desired mount (i.e. the skills and tools you have available).
Four materials are most commonly used to fabricate mounts; brass, steel, acrylic plastic, and aluminum. Each of these has different advantages and disadvantages and understanding these attributes will help guide your material choice and mount design. All of the metals should always have a barrier between them and the object.
Brass is the most frequently used mount material, due to its ease of working, its relative strength, and because it is fairly safe and non-reactive within a sealed case. It does not rust but will tarnish. Pieces can be joined together by silver soldering or by mechanical fastening. Brass can be easily formed with hand tools after annealing, and, if it becomes stiffer and rigid from work hardening, can be quickly re-annealed as often as needed to continue shaping your mount. However, unless very thick, brass does doesn’t have the rigidity to support heavier objects.
Steel is better suited for supporting heavy weights, being almost twice as rigid as brass. This increased rigidity also makes it more difficult to form, and may necessitate specific metal working tools; welding equipment, presses, brakes, and bending tools. Small pieces can be joined together by silver soldering, while larger pieces must be joined by welding or mechanical fastening.
Steel is also heavier than brass, but the increased strength it provides is often the more important factor for use with large, heavy objects. It can rust, so must be properly sealed before installation.
Stainless can be an alternative to brass for smaller mounts. Since it is also more rigid than brass, a smaller size of stainless used can be used than of brass. This may help in making the mount less obvious to the viewer.
However, being steel, it can still require specific metal working tools, and is also more difficult to form than brass. Small pieces may be silver soldered, while larger pieces must be welded or mechanically fastened. Most types of stainless do not rust.
Like brass, aluminum is easy to cut, drill, and tap with hand tools. It’s very lightweight for its strength, but, like brass, it’s not as rigid as steel. Additionally it’s not easily cold-worked and bending it can soon lead to metal fatigue, and breakage. It is one of the more difficult metals to weld. Lightweight aluminum panels can be very useful supports for large, light-weight objects.
Acrylic tends to be used because of its transparency and relatively easy working qualities, including shaping by heat forming; by hand, in a two part form, or by vacuum forming. Some downsides to its use are that it has a poor size to strength ratio compared to the other materials above, it is very reflective, and it can transmit light through from edge to edge. Although transparent, acrylic plastic is not invisible to the viewer. Nonetheless, being our only transparent mount material it’s often employed for that attribute.
Synthetic Casting Resins
There are a number of casting resins available that can be used to make part of a mount, greatly improving support for objects with irregular surfaces.
Sometimes called an interface, a resin cast conforms to a section of an object, allowing the mount to provide even support to the object and helps to avoid point-loading. A resin cast base or foot will greatly improve the stability of an object with an uneven bottom surface.
Resin casts can be easily cut, sanded, painted, drilled or tapped. The resin should be tested beforehand for user safety as cutting and sanding can send potentially hazardous dust into the work environment.
Resins are also adhesive, so caution should be observed when using them. Multiple layers of food wrap should be used as a barrier during casting. Be aware of undercuts in the object that might trap and lock in the casting. To avoid this you can fill the undercuts with soft materials in between the layers of the food wrap barrier to prevent the resin from filling in these areas.
Resin casts can also be used as part of an internal mount design, taking advantage of an objects irregular surface. The internal cast is made in sections, allowing it to be removed or reassembled inside the object. When installed it locks into the interior surface of the object, often negating the need for external restraints or clips.
Barriers and Padding
A barrier should almost always be used between the object and the mount, with the occasional exception of acrylic plastic, an inherently safe and non-reactive material. Polished acrylic can be used without a barrier to take advantage of its slickness, to avoid snagging when inserting it into a bag or article of clothing.
B-72 or several coats of Krylon Crystal Clear acrylic spray can be used as barriers, while other materials, such as acrylic or polyethylene suedes and felts or heat shrinkable or silicon surgical tubing can serve as both barrier and as padding.
It’s important to remember that any material in close or direct contact with your object should be Oddy tested first to ensure that it won’t react negatively with the object.
Using Different Materials in a Mount
Using different materials for their differing attributes within a mount will help your mount to perform as well as possible. You might use steel as a structural material, a resin cast to lock the object in position and brass clips to secure it from tipping out of position. Most of your well-designed mount will be hidden from view and any visible parts of it can then be made less obvious by painting to match or blend with the object itself.
A list of mountmaking supply sources is also available to download as a PDF.