Friday, September 23, 2011

Artist’s Journal: A Celtic Cross Commission

Exploring Shrinkage & Discovering the Power of Lavender Oil

by Yvonne Yao

For those of you who read my first project post on the “wingtip ring,” you will remember that my ring design turned out much smaller than I had intended. This is because, in my enthusiasm to create my design, I overlooked the importance of properly calculating shrinkage when it comes to working with PMC.

Recently, I was requested to design a custom Celtic cross for a client. I chose PMC as my creative medium, both because I do not possess the equipment to cast and forge in my studio and because of PMC’s wonderful sculptural attributes! My client had two requests, that the cross be specifically 1 3/4” in height and that it be engraved. So, I knew that I needed to apply some of my lessons learned from the last post.

Once I finalized my design sketch, I set to work by first creating a simple polymer clay model of the cross to get a feel for the height, thickness, and construction method. Satisfied that I had a sound approach, I double-checked my calculations. PMC3 has a 12-15% shrinkage rate and is one of the stronger fine silver clays. With that in mind, I calculated that my cross would need to be 2” tall before firing in order to achieve the finished 1 3/4” height.

I made a long PMC3 snake and cut the appropriate lengths for the cross and end caps. I rolled the remainder of the snake two cards thick to create the ring that backs the cross. I assembled the segments of the cross while the clay was still wet, by first moistening the ends to be connected and blending. When that was not effective I added some clay paste to help create a solid joint. However, in the future, I will probably experiment with more dry assembly since I think that will make the pieces easier to work with and support.

I allowed the cross to partially dry while I used a linoleum cutter to cut lines into the ring. Then I turned my attention to the surface, first sanding it to a smooth finish with 400 grit sandpaper, and then working back into the surface to create a rustic, uneven texture. I hoped that once oxidized, this texture would give the cross an age-worn, handmade look. Finally, I cut three groove lines into each arm to represent the holy trinity.

Once the cross was finished and textured, I mixed water into the fresh PMC3 clay to create a tacky clay-like slip to join the cross to the ring. I then finished the pendant off with a simple sterling silver bail, bent into the shape of a “U” and inserted into the top end cap of the cross 2/8” deep for stability. Finally, I wet the back of the cross and engraved it with the fine tip of an Exacto knife, finished it with light sanding, and fired.

Because I used sterling silver wire for the bail instead of pure silver, I fired the cross at 1300º F instead of the 1650º. This is because sterling silver is an alloy of copper and silver and an alloy has a lower melting point than pure metals. Firing at 1650º F would have melted the bail. So, I fired the pendant at 1300º F for 30 minutes.

[Ed. note: Firing for a longer time, at a lower temperature allows the metal particles to 'shrink' or sinter more closely together, creating a denser piece of metal.

The result was lovely! However, upon taking the pendant out of the kiln—I found fine hairline cracks along the center of the cross and a more significant crack along the base of the top end cap, where it connected to the body of the cross. Not un-fixable, but it definitely required some delicate repair and I knew it would need to be re-fired. This made me worried as to whether a second firing would cause the pendant to even shrink more.

With that concern in mind and some handy advice from Lora Hart, I had a decision to make. I could make my own clay paste with distilled water and fill in the cracks, leaving just enough clay on the surface to allow for shrinkage into the cracks. Or, I could use Oil Paste, which fires exactly as it is patched with no shrinkage. I didn’t have any Oil Paste, so I took Lora’s advice and stopped by Whole Foods to purchase a bottle of 100% pure lavender oil (no alcohol in the formula) to mix my own.

Using fine tip tweezers, I packed the homemade oil paste into each of the cracks until I was sure the crevices were filled and there were no air pockets. I sanded each patch to leave only the slightest lift on the surface so that I would not face a monumental sanding job later. Then I re-fired the piece at 1300º F for 45 minutes.

[Ed. note: When firing fresh clay to fired or milled metal, apply to the white, freshly fired surface or use sandpaper to give the metal some 'tooth' and always fire for at least 45 minutes to allow the fresh clay to bond more securely with the fired piece.]

Viola! The homemade oil paste worked beautifully! Even more of a relief, there was no additional shrinkage. The cross measured to almost exactly 1 3/4” in height. Hallelujah!

The pendant was then oxidized with jeweler’s black and finished with a green jeweler’s sandpaper to give it a final rustic touch. I hope you like the final result as much as I do.

See more of Yvonne's work here.


Tamara said...

Thank you for your wonderful article. It's packed with a lot of great information. Your cross is beautiful!

Roxanne Coffelt said...

I have heard that oil paste was the thing to use on PMC that was already fired, but have never tried it. Your article is very helpful. Thanks for sharing that!