Absolute cleanliness and a skilled hand are essential for the adhesion of cracks. There are two different variations. Firstly adhesion in situ, whereby the cracked piece remains in the leaded net in order to preserve it and secondly, adhesion in the disassembled condition. For very complicated cracks, adhesion requires a specially constructed surface that is adapted to the shape of the panel, e.g. in a sand pit.
The process starts with the careful cleaning of the cracked edges using a quickly evaporating, clean solvent.
The next step is to secure delicate surfaces and reverse sides against leaking adhesives. Lateral edges must be sealed with a special resin to prevent any adhesives from leaking into the lead and gluing together the lead and the glass.
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Measuring and applying of adhesives.
The correct choice of adhesive agent is crucial. ‘Araldite 2020’, an epoxy resin adhesive agent has almost always been used as standard, but other hardening agents, such as Acryl and UV are also in use. It is important to note that the hardening agent of the selected adhesive is not stronger than that of the glass, because the adhesive tension would cause the lead to break. Other criteria for the choice of an adhesive agent are durability, a comparable breaking index, minor discolouring and reversibility. The latter being an important feature in any materials used in the preservation and restoration of historic monuments and buildings.
The adhesive agent is applied in small dots along the edge of the crack and distributed throughout the crack by capillary effect. After the drying period, the residues must be removed carefully with a purified dissolvable solvent, before they harden on the surface.
Finally retouching may be done along the glued surfaces.
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