ASK THE EXPERT!
TOM ELLIS
Last week in the FORUM, Christoper Palko asked "Any success with Opalescence? Cracking with Thompson #2061 Opalescent white-
I forwarded the question on to Tom Ellis from Thompson Enamels and this is what he said....
When enamel cracks it is because there is too much stress and the glass has no choice but to relieve that stress by cracking. This can happen with any enamel under certain conditions. Some of these conditions could also lead to the enamel releasing from the metal or from itself, but the chipping defect may or may not involve conditions that always apply to cracking. Under the conditions of heat the metal expands. The enamel flows and is carried with the metal. As the piece cools the enamel becomes rigid between 1020˚F and 1050˚F.
The metal continues to contract. This puts a strain on the enamel and given certain conditions, the enamel will crack.
There are many reasons or combination of reasons why the glass/metal relationship fails. The reasons can include - the wrong alloy or base metal; no counter enamel; the wrong expansions relationship; uneven or too thick enamel application; uneven enamel thicknesses back to front;
non uniform base metal thickness; solder joints; weighting the enamel after firing; metal inclusions in the enamel; impact; the list can go on and on. There is not a general answer that will cover all situations. To understand what is causing the cracking involves looking at each instance of cracking individually, and looking at all the conditions that may be causing the cracking or contributing to it.
In order to solve a problem we start by asking questions. Answers need to be very specific and usually a good photograph can provide information that may not have been considered in the questions:
1. What metal is used? If an alloy, be very specific as to the alloy.
2. What size and shape is the piece?
3. What is the metal thickness? Is the base metal exactly uniform in thickness? Is the piece flat or domed? Is the piece cast, metal clay or from sheet? Is the enamel placed into a recess in the metal? If so, what is the depth of the recess? What enamel technique is being used? What is the method of application of the enamel? How does the thickness of the enamel compare with the thickness of the metal? What equipment is being used to fire the piece? What is the firing temperature? What is the length of firing time? How is the piece left to cool? Where are the cracks in relationship to shape of piece? cloisonne wires? etc.?
The expansion of 2061 is 343. That is on the high side of expansion for enamels for metal. In general you want the lowest expansion on top. This is not always the case, and seems most important when at extremes. 1020 is a low expansion white which if a high expansion transparent enamel is fired on top, cracking can occur. I have used 2061 on top of 2020 (expansion - 305) with no obvious problems but it could be my enamel thickness is on the thin side, so that too much stress is not created.
I include a photo of a piece that is 1-1/2 inches square, 18 gauge copper domed, counter enameled, base coated with 2020, silver cloisonne wire and silver foil, and the right-side whitish portion is 2061 over silver foil over 2020, the circle is 2061 over 2520 on 2020.
The 2061 came right out of current stock. There have been three firings at 1430˚F for 1-1/2 minutes. After the second firing the piece was stoned and re-fired. After the third firing the edges were polished with a medium Cratex wheel on a polishing motor.
There is no cracking of the 2061.
For the cloisonne in question, I would look at the enamel thicknesses as they relate to one another, and back and front; the thickness of the base metal as it relates to the enamel thickness; the expansion of the enamels used together; the cloisonne wires as they relate to the enamel areas; and consider the location of cracks on the piece in relation to the parts of the piece i.e. cloisonne wires. Try to imagine the metal expanding and contracting in relation to the piece and the cracks.
The greater the thickness of the enamel in relationship to lesser thickness of metal is worth considering first. The piece should certainly be counter enameled. Slow cooling is best as well as lower temperature which would obviously decrease the amount of expansion which decreases the amount of contraction.
Tom Ellis