Frequently Asked Questions
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Which adhesive was used to repair the vases?
The joins were made with a colourless 2-part epoxy resin (HXTAL™ NYL-1), a very fluid conservation-grade adhesive that crawls into very narrow cracks by capillary action. It does not shrink when it hardens.
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Epoxy resins are very strong, hard adhesives. Conservation-grade epoxies are suitable for repairing hard non-porous ceramics such as hard-paste porcelains. They are not appropriate for weaker or porous ceramics such as earthenware. HXTAL™ NYL-1 was chosen because it has better ageing characteristics than some of the other available conservation-grade epoxy resin adhesives. The cured translucent resin also has a similar refractive index to the glazes on hard-paste porcelain. It is this quality that makes cracks seem to disappear when it is applied.
The HXTAL™ NYL-1 is a 2-part liquid adhesive measured by weight in a 3:1 ratio. If not mixed accurately the adhesive will not set and the bonds may fail. The components were weighed out exactly, drop-wise on scales, before mixing thoroughly. A small metal sculptor's spatula was used to mix the adhesive and apply it to the taped cracks.
HXTAL™ NYL-1 has one principle drawback. The adhesive takes two weeks to harden. There is a risk that dirt will settle onto the wet surface, causing staining. To prevent this, the vase is covered loosely in acid-free tissue paper while it dries.
What are the desirable features in a conservation-grade adhesive?
- Bond strength appropriate to the ceramic to be repaired
- Compatibility (e.g. appropriate viscosity) with the ceramic to be repaired
- Good long term stability and reversibility, non-yellowing
- Good surface wetting (i.e. the adhesive coats ('wets') all parts of the surface to which it is applied
- Colourless ('water-white'), transparent
- Close match to the refractive index of any glazes
- Good working time
All adhesives will eventually discolour as they age, but conservators use those that remain stable for longer. In conservation timescales, good stability and reversibility means ideally using materials that will not discolour and remain reversible for a minimum of 100 years under museum conditions. This is not possible for most epoxy resins currently on the market. Most adhesives age and discolour faster if regularly exposed to strong light and heat, such as direct sunlight. Oddly, there is also evidence that some epoxy resins need some light to prevent them from discolouring, so repaired objects should not be stored in total dark, such as a cupboard.