Late Anglo-Saxon Disk-Brooches. Part 4 (Enamel)

Fig 1. Fired Hand-made Enamel Bezel Cups.

This is Part 4 of my series of blog posts related to my Late Anglo-Saxon Disk-Brooch Project. Please see Part 2 of my series for information on my reasons for experimenting with Flameworking glass rods and turning them into a fine powder which can be used as Vitreous Enamel. Part 3 covers my first series of experiments on breaking down the rod sections into smaller pieces and then grinding, rinsing, drying, sifting, and storing the fine glass particles of each color of glass. Part 1 is a general history of the disk-brooches that my research and fabrication project centers around.


A few lessons learned:

It's impressive the huge difference various tools can make when compared to one another. 


Contamination from the Marble Mortar and Pestle caused Light flecks 

My hand-ground Enamel was also contaminated with white and clear particles (Fig 1, 4, and 5). The significant differences can be seen between the bottom most red enameled bezel cup (Fig 1), which was ground only using the Stainless Steel mortar and pestle (Fig 2), and ALL of the other samples, which were ground mainly with the Stone mortar and pestle (Fig 3), which is possibly made of Marble, before switching to the Stainless Steel set (Fig 2). 

Fig 2.  Stainless Steel mortar and pestle

It turned out that the Stone's hardness wasn't as high as we thought compared to that of the glass being used. Unfortunately, I wasn't able to find, when I searched online using Google, what the range of hardness was for CoE 104 (Coefficient of Expansion) Lampworking 'soft' glass rods. Looking at the "Mohs Scale of Hardness", ". . . glass rates about 5.5, and a steel needle is a 6.5. Most Granites rate about a 7 in the scale while most marbles, limestones, travertines rate in the 3 area." [Source; accessed 2June2019], "Stainless Steel 5.5-6.3" and "Soda (soft) Glass 4.5, Glass 4.8-6.6" [Source; accessed 2June2019].

Switching to using a Stainless Steel mortar and pestle (Fig 2) made a significant improvement, both in the speed and ease of breaking and finely grinding the Flameworking glass rods into 80-mesh Vitreous Enamel. 

Fig 3.  Stone mortar and pestle.

Theophilus, in his treatise, "On Diverse Arts", doesn't specify which materials to use for either the mortar or pestle. This is most likely due to him expecting his contemporary reader to know and own the appropriate one(s). 

Benvenuto Cellini advises in "The Treatise of Benvenuto Cellini on Goldsmithing and Sculpture", "a little round mortar of well-hardened steel, and about the size of your palm...with a little steel pestle specially made for the purpose of the necessary size." (Fig 2)

I had started to grinding all of my Flameworking Glass rods with the Stone mortar and pestle (Fig 3) which looks like white 'marble' with pale grey veins. I found that grinding the CoE 104 Flameworking glass was quickly causing wear and tear to the mortar, but especially to the bottom concave curve of the pestle. 

Switching to only using a Stainless Steel mortar and pestle (Fig 2) made a significant improvement by eliminating additional contamination from the Stone mortar and pestle's surfaces (Fig 3).

Unfortunately, rinsing out the ground up glass multiple times did not remove the contamination, it stayed and caused, at a minimum, color issues and a great deal of very fine white speckles throughout all the colors, as can be seen in the photograph (Fig 1). I do not know what possible chemical interactions could have occurred during the firing that reached 1,500F.

Fig 4. Rows #1 & #4 are Stoned,
The photos are at two different angles.

I started to stone the highest surface with 150 and 220-grit Alundum Stones so that the glass and metal would be uniformly level (Fig 4. Rows #1 & #4 are Stoned in all 5-colors). Quickly it was obvious that additional enamel layers needed to be added so that the center of the concave glass, concave meniscus (Fig 4), could be filled in and raise closer to being level with the bezel cup's walls. This would help reduce how much metal and glass would need to be stoned away to complete the stoning stage.

Once I've added sufficient layers of enamel I will need to finish: stoning, pickling (pickle is a mild acid solution that removes oxides from the oxidized Sterling Silver), flash fire (to make the glass shiny once more), and polish the Sterling Silver bezel cups to complete the process.


The importance of using the correct temperatures

Fig 5. Melted Bezel Cups

The hand-ground Lampworking (soft) glass that I made in five Anglo-Saxon colors, which has a CoE 104, was sifted so that it is the standard 80-mesh size that Thompson Enamels sells. This hand-made enamel was wet packed in multiple thin layers within the Sterling Silver Bezel cups and fired each time an additional layer was added. The Bezels melted in the Kiln when it reached approximately 1,700F, it is usually set to 1,500F (Fig 5). 

If Copper and either Fine Silver (99.999% Cu) or Sterling Silver (92.5% Ag and 7.5% Cu) are physically touching one another when they reach their melting points then the Silver will look like it is melting into the Copper or look like it is being absorbed by the Copper. The Silver (Ag) atoms slide within Copper's (Cu) crystal lattice. Silver and Copper are Eutectic [also called, Eutectic System].

This can be prevented if there is a layer of glass / vitreous enamel between the Copper and Silver, *Enamelers call it 'Flux'. As soon as the glass is thinned away over several firings and leaves a bare spot, during the needed temperature range, then the Silver atoms "slide" into the Copper (Cu) crystal lattice. 


* 'Flux' in Enameling is clear glass enamel, but it should NOT be confused with what Metalsmiths are referring to as Flux, which can be made in different ways, but usually it's a solution of Borax mixed with water. Flux (Borax, etc.) helps prevent oxides on hot metals from forming, molten metals flow better, the solder binds to the metals and flow more easily.


Silver and Copper has are Eutectic [also called, Eutectic System], both elements are Face-Centered Cubic (FCC) structures (scroll down to see a 3D image of the structure; "The face-centered cubic (fcc) has a coordination number of 12 and contains 4 atoms per unit cell." Source; or for a digital animation.)

Cu/Ag Eutectic System, "Copper and Silver are both FCC, but their lattice parameters and atomic radii are very different, so they have limited solubility in the solid state. There are two solid stable phases α and β, and at high temperatures there is a eutectic reaction where the solids α, β and the liquid coexist.", "Cu – Ag System, Cu: α phase, Ag: β phase", "Eutectic means “easily melted” in Greek." [Source; see slides 1-4]


Contamination from the Copper sheet causing Dark flecks

Fig 6. Flaked off Oxides from the Copper sheet that was
used as a support for the bezel cups during the kiln firing.

The dark flecks in the fired Enamel Bezel cups are from the flaking off of oxidized layers that were formed on the Copper sheet during the high temperatures reached within the kiln. 

I used the Copper sheet within the kiln to support the small Bezel cups during the firings. The metal mesh screen that is usually used on it's own to support Enamel pieces didn't properly support the Bezel cups so that they could remain flat (on the Left of Fig 5; Fig 6). The flaking black oxide layer can be seen underneath both the melted and whole bezel cups (Fig 5) and in the pile accumulated after the firings (on the Left of Fig 6). 


My Video of the Enameled pieces being removed from the kiln and cooling (on Facebook)

My short video is 3m25s long and shows a kiln firing of the hand-ground enamel I've been making from Flameworking glass rods.

The target kiln temperature is 1,500F for my enamels. At the start of the video the temperature is 1,450F (it might sound like I said 450F, but it's 1,450F) I open the kiln door to allow some of the built up hot air to vent out and get the kiln down to 1,250F. Once it's reached 1,250F (not 250F as it might sound like in the video) I gently place the Stainless Steel sheet, that's resting on the steel mesh frame, onto the kiln floor and close the door.

As soon as the temperature reaches 1,500F I carefully open the door and remove the sheet and metal frame with a pair of long pliers and place them gently on top of a ceramic tile. At the same time I'm wearing one heat protection glove on my dominate right hand that's holding the pliers, the heat is so high that even 5 to 10 seconds of exposure on my skin starts to sting.

The pieces change colors as they cool down from 1,500F. Once they are completely cooled they can be moved and worked on.


Part 5 of my series of blog posts will be about the results of firing the newest batch of Enamels that I've I made using just the Stainless Steel mortar and pestle. Once again I will not be mixing my Enamel with my Thompson Enamel powders due to the different CoE which could cause issues.