*nb: the title of this talk, the author list, and author order all changed from what was published in the program. If I have left out an author name, I apologize.
The focus of this talk was to discuss the application of atomic layer deposited films of aluminum oxide (Al2O3) onto sterling silver surfaces. Ultimately, it is hoped that this process will be refined for use with sterling silver art objects in museums.
The overarching goals of this project are to find a coating for silver that is long lived; slows the diffusion of sulfur to the surface of the art object (I learned this fact at the presentation by Zeev Rosenzweig at the RATS luncheon); can be rapidly applied and covers the object homogeneously; is removable; and is less labor intensive than manual application and mechanical polishing.
(wow, that’s a lot!)
The University of Maryland has a “BENEQ TFS 500”, a commercially available atomic layer deposition (ALD) machine/instrument, to deposit thin film coatings on anything that is placed inside of it.
Awesome facts of this instrument:
• Samples placed in the instrument can touch one another and it won’t affect deposition;
• You can coat MANY samples all at the same time
• You can control the thickness of the layer you deposit onto the objects you place inside the instrument, down to the nanometer (nm) thicknesses
• Coatings are incredibly uniform
• The films made are dense (ie, tightly compacted), and these types of films make good vapor barriers
• Coating will go into very small crevices and tool marks
• Tailoring thickness allows you to control optical properties
• It can take a mere 1 hour to make a coating that is 100 nm thick
The project is currently examining whether aluminum oxide coatings perform better than nitrocellulose ones. They used accelerated aging studies (40°C, 30-50% RH, 20 ppm hydrogen sulfide pumped through a sealed chamber) on coated samples and performed Tarnish Rate Analysis (never heard of this!! measure color of tarnish; correlate to thickness of tarnish layer), X-ray photon spectroscopy (XPS), secondary ion mass time of flight mass spectrometry (SIMS-TOF), and atomic force microscopy (AFM) to analyze this.
RESULTS and CONCLUSIONS:
* A 80 nm aluminum oxide coating outperformed a microns thick nitrocellulose coating because it is more tarnish resistant than nitrocellulose.
* They said that that these results are very fresh and new but that the aluminum oxide coatings are removable with a minimal loss of surface copper and silver using dilute sodium hydroxide (NaOH) but that you have to be careful about the concentration of the NaOH because it can selectively remove copper over silver from the sterling silver alloy itself. But that we should all relax because the amount of copper or silver being removed is on the nanoscale.
Comment: Eric presented removability study with weak NaOH compared to data from Glenn (I don’t know if this work is published or has been presented elsewhere), who used three different chalks (all of which were in the lab);
Q: is this atomic layer deposition instrument expensive?
A reactor costs…$500K. BUT you can build one or buy the parts separate and assemble one yourself; not rocket science to build one; there is a place in Cambridge (MA? England?) that sells on for <$100K; you can make a homemade one for ~$50K
Q7: how long will coating last?
At least 20 years; a guess, no hard evidence
One facet of this project is to build coatings even better than aluminum oxide
TERRY Weissman: lot of the current results presented here are theorhetical; we will be getting into more of the conservation issues as time goes along;
Also, we knew that NaOH would affect the copper; but often times, art objects are already Cu depleted; We might find a better solvent in the future