electric heating elements in order to produce an even temperature distribution
within the retort. The reducing atmosphere was obtained by passing ammo-
nia over a nickel catalyst at 850°C to produce an atmosphere of 75% hydro-
gen and 25% nitrogen. This mixture of gases was then fed into the retort at a
slightly positive pressure and out at the bottom of the retort and exhaust for
analysis and extraction of harmful constituents.
Initially, ferrous artefacts were stored in 2% sodium hydroxide to prevent
any further corrosion prior to treatment in the hydrogen furnace. It was sub-
sequently found out that the sodium hydroxide converted some of the chlor-
ides in the rust layers to sodium chloride. This was only very slowly removed
in the retort as it does not sublime at the operating temperatures or get
reduced by the hydrogen to release the deleterious chloride ion. Ammonia-
based electrolytes were subsequently used for pre-storage prior to the artefact
undergoing hydrogen reduction. The chlorides are now converted to ammo-
nium chloride, which sublimes off at temperatures in excess of 520°C
and undergoes reduction in the hydrogen atmosphere present within the
retort.
After removal from the soaking tanks, the concretions were removed from
the artefacts. For large items such as the guns, this was mostly carried out by
mechanical methods, using chisels or scalpels. Light concretions on small
artefacts were not touched as they were either removed during conservation
or were easily brushed off after removal from the furnace. The final pre-
treatment were to make quite sure that the barrel of the guns was clean so that
the atmosphere within the furnace could freely reach all parts.
Once cleaned, the artefact, if it was a gun–barrel, was secured onto a stain-
less steel base (Figure 12) and the Nimonic alloy retort clamped down over it
so as to ensure an airtight seal.
Small artefacts were placed onto stainless steel mesh trays and also fixed into
the retort so as to give maximum circulation of the furnace atmosphere during
the conservation process. The retort was wheeled into the electrically-heated
furnace purged and with nitrogen until the last traces of oxygen were removed.
The cracked ammonia replaced the nitrogen as the atmosphere within the retort
and the temperature of the furnace was allowed to rise slowly to the operating
temperature over a 24 h period. For cast iron and most artefacts recovered from
marine sites, this would be 850°C. This was to ensure that the artefact was not
subjected to any significant thermal shock. The temperature would remain at
this value until the chlorides in the exhaust gases coming out of the furnace were
below 10 ppm as detected by Draeger tubes. This could be anything from 100 to
200 h depending upon the original levels of chloride within the rust layers.
The cracked ammonia was replaced by nitrogen in the furnace and cooled
down over a 48 h period. The artefacts were removed from the retort and given
a very light brush to remove any powdery deposits left on the surface. The
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