28 March 2013 - Coin Hoard Blog - Jersey Heritage

Last week I went ahead and did the moulding of the hoard's surface in order to produce a replica.

Last week I went ahead and did the moulding of the hoard's surface in order to produce a replica.  I did it in the same way as the test piece described above, the only difference being that I inserted a flexible plastic foam into any crevice that would have "trapped" the rubber too much and caused damage on pulling it away.  For instance I filled the inside of the gold "bracelet" loop and behind the gold torc.  When the replica is cast I will grind away the replicated foam areas to match the original surface.

In the end the best way to do the whole surface proved to be in three sections.  I did the entire top surface of the hoard in one go, making sure that I overlapped the side wall edges slightly.  Before it set I inserted a long strip of 150mm high clear plastic sheet into the rubber all around the hoard's top edge.  This acted as a wall all around the top of the hoard.  This was needed because I had decided to use polyeuethane foam as a backing for the rubber.  A backing is needed because although the rubber preserves every detail on the hoard's surface, it is very flexible and floppy.  By itself therefore it wouldn't preserve the hoard's shape well.  When it sits on a hard foam backing that is moulded in place however it is rigid and accurate.  The foam is made by mixing two resins and pouring them onto the rubber, allowing the mix to then start bubbling, expanding and then hardening.  Doing this in a series of small stages I built up a 100mm thick rigid foam backing.  Once this was hard I then pulled it off the rubber and then carefully peeled the silicone rubber sheet off the hoard top.  Pulling it off in this way, however carefully, did dislodge more coins than I was happy about but each was placed back on the surface where it came from.  Although the process has changed the surface of the hoard slightly the replica produced from it will preserve the original surface perfectly for all time.

Having done the top surface I then went onto the sides.  I did about 80% of the edge in one long rubber strip, leaving only the area with the exposed gold torcs.  I inserted a protective polyester wall again to protect the top surface coins and I then backed the side in foam in the same way as the top.  I later cut the hardened foam off in four sections but left the rubber in one piece which I peeled off as before.  When I come to do the casting I intend to make the edge strip in one go but will do the work in four sections, each bit suppported by the appropriate foam backing.  The removal of the rubber did little damage except for one area to the left of the gold pieces where a section of the surface about 2cm deep and 5 by 10cm wide did fall away.  The only remaining area left to do was the one with the gold torcs.  I had initially thought that the rubber removal would be too damaging to allow its use here but I worked out a safe way to do it.  All the areas so far had been given a deeply penetrating thin liquid coat of silicone rubber and it was this penetration that caused some damage on removal.  For the gold area I did not use the thin liquid rubber but went straight onto the thickened paste-like mix I had previously used for backing the first layer.  This paste was applied with gloved fingers and a small  trowel and was not pushed deeply into the surface.  This led to some compromise of deep seated detail but allowed easy removal with no damage to the jewellery.

Pulling all the rubber strips and sheets off was the scariest thing I've done since we lifted the hoard out of it's original hole in the field.  I'm pleased with the results however and will start producing the epoxy cast after easter.