Hexlite 620 (formerly Aerolam F) was developed for the aerospace, automotive and marine industries, but it is of great use in conservation as a support material, and as a lining or backing for paintings, for example. It is an Aluminum honeycomb panel or board, composed of an Aeroweb Aluminium honeycomb core sandwiched between an Epoxy-reinforced Glass fiber skin. The board has a cream skin with the aluminium honeycomb core exposed at cut edges and is textile-like and smooth in appearance. The skins are comprised of two layers of 10 ounce woven glass fibre sealed with an epoxy resin, the woven structure giving multidimensional strength. It is available in various thicknesses of 13.7, 26.4 and 52.3 millimetres, and is manufactured in standard sized panels of 4 by 8 feet (2.44 x 1.22 m).
The Aluminium core has a density of 83kg/m3; its honeycombed structure means that the board has incredible dimensional strength and is lightweight, having typical weights of 3.08kg/m2 (13.7mm), 4.21kg/m2 (26.4mm) or 7.54kg/m2 (52.3mm). Mechanical properties and Temperature resistance have been tested and proven by the manufacturer; the product has shown to have a low moisture pickup of less than 2 percent at 95 percent Relative humidity and it is resistant to extreme temperatures. It can be used in conjunction with a variety of different adhesives, ranging from acrylic dispersions to heat-seal adhesives such as Beva 371, but it is more commonly used with epoxy resins. Epoxy resins are usually used to adhere mosaics and heavier objects to the board because they impart greater strength.
Hexlite 620 can be easily mounted to walls using screws and battens, providing the object is not too heavy. This is particularly useful for wall hangings and paintings. For heavier objects, such as mosaics, the board provides a lightweight and strong method for mounting them vertically on a suitable frame. Bolts can be adhered into the aluminium core of the board using epoxy resin, so that it can be fixed to a frame. Unlike horizontal display, this saves space and allows for ease of viewing.
The primary use of Hexlite 620 in conservation practice is as a support or mounting material. Mounting and backing is an essential part of an object, both in aesthetic and structural terms. When backing heavy objects such as tiles and mosaics, one of the primary criteria for the backing material is strength and rigidity, whilst remaining lightweight for ease of handling and transport. It is essential that the support does not flex. Hexlite 620 fulfils these criteria. Board thickness is chosen according to the load that is to be placed on it and the size of the object. Thicker board is required for heavier objects, but large mosaics may require two layers. It is essential that the load and size of heavy objects is used to calculate the thickness of board required, especially if they are to be displayed vertically. In some cases, a frame is necessary to add extra support. Strips of Hexlite 620 can be adhered around the edges of the board and a frame applied for greater strength. Frames are sometimes necessary for large or heavy objects to give greater strength, and also for traveling exhibitions where it cannot be guaranteed that the display surface will be flat enough to accommodate the Hexlite 620 panels.
Tiles have been successfully mounted on Hexlite 620, but it is often necessary to apply a separating layer between them and the backing to ensure reversibility. One method for mounting tiles involves drilling holes through the board where adhesive is to be placed at the corner of each tile. The same method was used in the reconstruction of an Iznik tile fireplace . This ensured that the backing could be easily removed in the future when Cotton wool saturated in an appropriate Solvent is placed over the holes, allowing the fumes to penetrate and soften the adhesive. With larger panels and objects such as mosaics, removal of the backing may be more challenging. This is because side access is limited during reversal procedures, and there is a risk of damaging the object when using mechanical means from the sides or front. However, the board can also be dismantled mechanically and epoxy resin can be dissolved in Dichloromethane if necessary, but lining textiles and paintings on rigid supports is not easily reversible unless a barrier is placed between them.
In stone conservation, Hexlite 620 has been successfully used to support broken slabs. The material has major advantages in the respects of minimum intervention and offering a lightweight support of uniform thickness. However, it has limitations because it alters the thickness of objects. As with many backing materials Hexlite 620 obscures the reverse of an object, but it has an advantage in that windows can be cut into it to allow any important information on the reverse of objects to be viewed if necessary.
As Hexlite boards are smooth, they sometimes require roughening through light abrasion in order for any adhesive to key to their surface. Polyvinyl acetate impregnated bandages can be adhered to the board to provide a surface onto which a Plaster filler can key. When displaying heavy objects vertically, surely the strength factor of the adhesive used and how effectively it keys to the surface of the board should be of primary concern. The surface of the boards should be cleaned free of Grease and Oil to ensure a good Adhesion; vacuuming should also be undertaken to remove Dust. The success of Hexlite 620 as a conservation material is dependent upon its preparation, fixings, and the preparation of the object requiring support.
Many objects requiring mounting often have an irregular reverse, so the flat nature of Hexlite 620 can present problems. With objects such as mosaics, a flat reverse is essential to adhere the Hexlite to because it provides the load bearing support. The board can accommodate a gentle curve but will not go into depressions, which can pose limitation in the backing of irregular wall plaster, for example. Tiles also need to be of a standard thickness to ensure a uniform relaying.
It is sometimes necessary to use a custom made backing, or spacing boards tailored to the shape of the object prior to mounting on Hexlite 620, in order to avoid stressing an object. Although these methods can be seen as a drawback, it does mean that a wider range of adhesives can be used, including those that are irreversible, to adhere the object to the Hexlite board because of this isolating layer between them. This can be particularly useful when mounting paintings. Polyethylene foam coated in Paraloid B-72 adhered to Hexlite 620 has been used as a barrier between the board and a painting, thus ensuring an easily reversible backing. Reversible adhesives are often applied as separating layers to the reverse of tiles and mosaics to allow the Hexlite backing to be removed in the future. The board allows fragile objects such as paintings and mosaics to be handled and framed with a reduced risk of damage due to its rigidity, which means that these objects can be transported safely between exhibitions and museums.
Textiles can also be mounted on the board after covering it with an appropriate fabric. Fragile silk has been successfully sandwiched between the board and an Acrylic sheet, firmly screwed in place. Hexlite panels are thus advantageous in preventing stresses from rolling and hanging. However, environmental fluctuations may cause movement in organic materials and the rigid nature of Hexlite 620 does not allow for this.
Hexlite 620 has also been very effectively used to fill missing areas on stone objects. Hexlite 620 proves ideal because each piece can be tailored to fit missing areas, and a grade can be chosen to fit the depth of the stone. The board can be attached using a paste of epoxy resin and filled over to match the sandstone. It has an advantage because it can be painted if necessary.
The board is easily cut using a hacksaw, but for more complex shapes a power jig-saw is more practical. Due to the rigidity of the board and the cell density, it can be cut without deformation. The material has advantages in that it is non-toxic and inert. However, dust from cutting the board is dangerous by inhalation and aluminium dust is combustible, so dust extraction and protective clothing is recommended. Sharp aluminium and glass fiber is exposed at cut edges, so gloves should be worn during its preparation. The rough edges are usually sealed afterwards, which has been done in the past with gummed linen tape and wooden trims. However, organic gases may corrode aluminium, so materials such as wood should be used with caution as they can release such gases. Polyfilla has also been successfully used, and Plastazote® or aluminium strips attached to prevent damage to vulnerable edges. However, the PH of sealing materials should be considered, as well as Moisture exposure when using materials such as Polyfilla because moisture may become trapped within the honeycombed structure for some time, both of which may induce Corrosion.
Should a sheet larger than is commercially available be required, the construction of the joints used to fabricate it can become fundamental to the strength and effectiveness of the backing. The manufacturer often recommends methods of joining sheets together, but these are intended more for industrial purposes and may be unsuitable for conservation. An adhesive of suitable strength needs to be chosen, such as an epoxy resin; an Iznik tile fireplace was reconstructed for the Victoria and Albert Museum , where complex shapes of Hexlite 620 were cut with mitred edges and attached together with an epoxy resin. The joints were then coated with epoxy-impregnated glass fibre matting to give added strength. However, it is not always desirable to mount objects on boards larger than their standard size due to issues of transportation; multi-section supports are often more practical because they can be safely and easily transported, stored, and assembled on site. Hexlite 620 has an advantage in that it can be cut to the required dimensions, and a border can be left if required to facilitate handling or protection to an object during storage.
Resources and Citations
- Contributed by JPeake (talk) 11:52, 3 September 2013 (CDT)
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