BTA Treatment of Bronze Figurine

I finished treating the Greco-Roman bronze figurine of a bear or wolf on top of a man. I scraped off all the active corrosion, getting as close as possible to the copper but not scratching it. The worst places were underneath the animal and in the incisions on the man. Unfortunately, it was quite hard to get the scalpel into these locations. Although I was able to remove most of the chlorides, there are probably still some spots of corrosion. It is hard to remove them all by hand, even using a small scalpel and microscope. The corrosion layer that remains is not active, and serves to protect the metal underneath. This layer is green and orange on my figurine, whereas the active corrosion was whitish and light green spots.

Satisfied with the results, I inhibited the bronze figurine to hopefully prevent future corrosion. To do this, I immersed the figurine in a 3% solution of benzotriazole (BTA) in ethanol.

BTA is used to coat industrial copper, just as pipes, to prevent corrosion. There is debate over its usefulness in conservation, as the old copper alloys treated this way generally already have corroded. BTA chemically binds to the surface copper, creating a barrier between the metal and the moisture of the environment, but corrosion underneath can erupt through the coating. However, it does seem to slow down the deterioration.

BTA comes as white, long pellets. In general, BTA is dissolved in water or ethanol at 1-3%. I dissolved 30 grams of BTA in 1000 ml of ethanol. This was done by gradually adding the pellets to the cold ethanol and stirring, but some people heat it and apply it hot to make the entire process faster. The solution can be sprayed or painted on, though the most effective method is to submerge the piece for 24 hours. The conservators I work with chose to use the higher percentage and reduce the time in the bath to just a few hours. In this case, I left the figurine in the BTA for four hours.

I took out the bronze figurine and let it air dry. After this treatment, gloves should be worn when handling the figurine. Occasionally, excess BTA dries on the surface as a white crystal, but that can be brushed off or wiped off with ethanol. The white patch that appeared on mine probably occurred from bleaching the adhesive used to put the accession number on the figurine. Sometimes the copper alloys are coated, such as with wax or lacquer, but the Kelsey's collection is in a stable environment so this step is unnecessary for my piece.

Consolidation of Ceramics

Today, I consolidated both the Islamic bowl I have been working on and a second Parthian jug.

The jug is larger and thicker than the other jug I did. I had previously cleaned and desalinated the ceramic. It took a long time to desalinate, over two weeks. There are still some soluble salts in the fabric, but the conductivity plateaued. The jug is unglazed and fairly porous. Although a couple pieces are missing, for the most part it is whole, broken up into two dozen pieces.

I used the 10% solution of Paraloid B-72 in acetone that I had made up last week to consolidate the break edges. Using a small paint brush, I sealed all the edges of these two ceramics, which took several hours total. The Parthian jug was easy. The break edges were wide and relatively cleanly broken. The Islamic bowl was harder. The pieces are quite small, and I only wanted the consolidant on the ceramic, not the glaze. When some went onto the glaze, I cleaned it off with acetone. For the laminated pieces which are just one sided sherds of glaze, I consolidated the non-glazed side.

The Paraloid B-72 dried quickly. Some of the ceramic edges look slightly darker than they were, but otherwise they appear to be the same. They feel slightly smoother now though. The consolidation will prevent the ceramic from soaking in the stronger adhesive that I will use next week to join the objects and will protect the pieces from crumbling. It will also make it easier to take apart next time if someone needs to treat it further.

Cleaning the Islamic Bowl

It has been a long time since I last posted, but I have been doing the same project since February. I had a six week break when the conservators went to Egypt, but even so this one project has taken a very long time. I took apart the Islamic bowl and removed all the old gap fills and plaster. Many of the pieces in the central section were laminated. The front and back of these pieces were stable thanks to the glassy glaze, but the porous ceramic in between deteriorated. This resulted in the ceramic crumbling out, leaving two slivers of glaze. At some point, the front and back pieces were sandwiched together with plaster. I took apart all the pieces, including these sandwiched pieces, and removed the plaster. This resulted in 215 pieces of original ceramic, and perhaps as much as a third of the bowl missing.

Three of the five sections. Top right: the top are the original sherds and the bottom are plaster gap fills. Bottom: the plain gray pieces are the backs of painted sherds which have separated.

After removing all the plaster from these pieces, I then had to remove all the adhesive left on the break edges. There seemed to be a couple types of adhesive. The first was water soluble. When I soaked the pieces in warm water to remove the plaster, this adhesive easily dissolved. I just had to brush the break edges to remove it all. However, after drying, a white, solid adhesive appeared on many of the pieces when I checked them under a microscope. It could have been swelled and turned milky due to the water, which is why I did not see it before I started cleaning the pieces.

To remove this harder adhesive, I applied a 1:1 solution of ethanol and acetone to the edges with a cotton swab, which turned the white adhesive slightly 'gummy'. Using a microscope, a scalpel, a pick, and a brush, I was able to scrape off the adhesive. It was hard work as it did not want to come off. This stage of the process took up most of the time I have spent working on this bowl. This week, I finally finished.

Next, I need to consolidate the pieces. This involves painting on a 10% solution of Paraloid B-72. The weak adhesive will protect the pieces, sealing them and stopping them from crumbling further. I made up the solution this week to use next week.

Paraloid B-72 comes in solid, clear pellets. I measured out 20 grams of Paraloid and just under 200 ml of acetone. Technically, a 10% solution needs a 1:9 ratio, but Claudia said she tended to measure out a 1:10 ratio because the solvent evaporates and on excavations it is easier to round up. I then wrapped the pellets of Paraloid B-72 in gauze and tied it with thread, creating a bag. This was placed in the jar of acetone so that the bottom touched but the majority was out. By suspending the bag, I did not have to keep shaking or stirring the jar so that the adhesive would not just settle on the bottom. The jar is then left for 24 hours to allow the adhesive to fully dissolve. Next week, it will be ready to use.

 Top left: Measuring out the Paraloid B-72. Top right: Suspending it in acetone. Bottom: Paraloid dissolving in the acetone.