The focus of this presentation was to address problems associated with the use of slow and quick setting silicone rubbers for objects conservation applications. The main problem is that silicone rubbers often leave behind liquid and solid residues on objects after they are used. While it may be possible to physically remove the solids, the liquid residues often result in the development of a stain on the surface of the object where the silicone rubber had been applied. The question becomes, what is the chemical make up of these stains and can they be removed using solvents?
Why are we using these silicone polymers in the first place?
• to make casts for recasting
• to allow you to see an image or tool marks inscribed into a object better than you can see them on the object itself (silicone rubbers are homogeneous in color while objects themselves may not be)
• to non-invasively investigate the surface topography of the artifact
The research project entailed selecting eight different silicone rubbers and applying them to a series of different surfaces (plaster discs, limestone, and glass). Following removal of the silicones, attenuated reflectance-fourier transform infrared spectroscopy (ATR-FTIR) was used to determine the chemical make up (O-Si-O and Si-CH3 bonds) and depth of penetration of the liquid residues left behind. Several types of mass spectrometry (evolved gas, pyrolysis gas chromatography [PYGC-MS]) were used to study the eight rubbers and the molecular weights of the materials that are excreted out of the silicone rubbers and left on the different substrates.
RESULTS AND CONCLUSIONS:
* Hexane can be used to remove some of the staining material. I wasn’t sure how this conclusion was drawn? Regardless, it was a better solvent than either methanol (or was it ethanol) for removing some of the residue.
* When residues were observed, the color of residue was related to the color of the mold/silicome material
* Residues are mainly high molecular weight (HMW) components of the silicone rubber and are in their polymeric form – this wasn’t necessarily an expected outcome but it was what was observed
* It wasn’t possible to do a quantitative comparison of the amount of residue left behind between different rubbers
* Slow setting silicone rubber is bad for porous surfaces and residues can penetrate up to 100 microns into the surface (depending on the porosity of the object)
* Given the same rubber, the amount of residue left behind is a function of the material its applied to (intuitive?)
*Siloxane residues are comparable for quick setting rubbers
*Residues maybe reduced, but it is unlikely that they are effectively removed even with non-polar solvents
* There were a couple of others, but future work is to see if barrier coatings (methylcellulose) will help prevent residue deposition.
COMMENTS made following the presentation:
Baltimore museum stopped using silicone rubbers years ago; using silicones on rubbers causes patinas to disappear – patinas were saturated with silicone residues; little bits of set silicone were stuck in interstices of porous surfaces;
• I was surprised – atr is not really ideal for depth of penetration analysis; I also don’t think you can examine an area smaller than 20 um wide; can be hard to measure depths of penetration very accurately)
Any idea how porous the plaster is? Gypsum plaster – can you measure porosity?
Made sure used same batch of plaster throughout the experiments