there are tensions anyway and   you must counter enamel   each time  you can . the metal must be really thick not to need it

I routinely enamel of 24/26 gauge but always counter enamel and never have a curl problem

Proving that if the substrate (base) stays straight, enamel will not crack when cooling.

I do a lot of arts professionally, including both metal/silver smithing and glass fusion.  so I know how materials move and behave with heat.  Enameling is newer for me, but I still carry over the understanding about working both with warm glass and metal.  Recently I acquired some fine silver coins and used some of them to make a sheet of fine silver sheet which I've been using to do some cloisonné with transparent/translucent enamels.  I did not counter and felt that I did not need to counter as the silver is staying flat after firing the enamels.  I've fired this particular piece some dozen odd times now.  After I placed and then filled the cells, I then filled in the background with Thompson Rose Pink.  And the enamel cracked everywhere that these was the rose pink color.  Frustrated I decided to try a warm glass or glass fusion technique of slowly cooling the piece.  So I returned the enamel back into the kiln and allowed the temp to come back to 1450 degree F.  After about a 5 minute soak, I simply turned the kiln off and let the piece slowly cool insider the kiln.  This is similar to at we do in glass fusion, allow the piece to cool over an hour or so.  With glass fusion the rule is to keep the piece in the kiln until it cools to 500 degree F.  In this case I allowed the piece to cloisonné piece to cool to more like 150 or 175.  This time there were no cracks under the rose pink.  And again, the piece remained flat through all of this.  I was using approx. 20 g for the fine silver.  So the issue seems to be not related to the COE variance between the metal and the glass (enamel) as the sheet remained flat, but was related to the thickness of the glass and the shock cooling which is typical of enameling where we take the piece out of the kiln after it reaches soaking temp without any slow cooling.  This being the case, I now think that I'll enamel a group of pieces, and not worry if they crack.  But at the end of the work day I'll refire all of the pieces an allow them to kiln anneal and slowly cool as the kiln cools.

Richard

I can see your reasoning

However I feel Trish also has a point, when the glass is still viscous it will not be stressed, however once the glass has hardened sufficiently to be capable of being strained then depending on the configuration the metal substrate could exhibit a range of different temperatures for given temperature of glass dependent on the nature of the cooling.

The simplest picture is that the glass and metal are at the same temperature, a slightly more complex picture is that the glass and metal are at different temperatures an even more complex picture is that for a given glass temperature the metal could exhibit a range of different temperatures dependent on how it is insulated.

Glass is a reasonable insulator.

I think your reasoning would be completely correct if the workpiece was cooled in a controlled fashion, perhaps by slow temperature ramping the kiln down -if we can cool slowly so that glass and metal are always at the same temperature then the only factors that really count are the differences in COE and the strength of the metal substrate. A thick metal would not bend and the enamel would either tear itself apart or it would not (dependent on COE mismatch) - the counter enamel would make no difference as the kiln ramping is controlling the temperatures and preventing any temperature gradient front to back.

I also enamel steel (without counter enamel) and have had no issues with bending - steel is stronger than copper and secondly it has a COE much closer to glass than copper, in some respects steel is a much more suitable metal to enamel than copper.

In short I think there will be situations where Trish's argument applies most strongly and others where your reasoning will apply.

Hi. From what has been said: Counter enameling keeps the work straight. So, in a manner of speaking, you can substitute the conditions we are investigating (unbending substrate) by having the base that is stiff and unbend-able by the glass.

You only would get a problem with a stiff base (glass cracking) if there is:

* too great a disparity of COE between glass and metal

* too rapid cooling

So, the key would be to seek to match, as best one could, the COE of the substrate and the glass - and to get the proper temperature program set up, really the right cooling program.

So, I'm getting that *in principle* it's alright to enamel on stiff substrates, as long as things are thought out carefully and use of trial and error.

Actually, when you think about it, the artist who works in flimsy copper and counter enamels, and the artist that uses stiff unbend-able substrates are both working with unbending substrates. So are sort of on a par in that respect. Except that one could be enameling on stainless steel, and that introduces a difference in experience, the parity goes, because of the potential problem caused by COE issues.

I can't see how I have missed this discussion. Read my article steel preperastion Notebook #1, liquid enamel part way down in the center of the page. This cleaning process allows me to lightly ground coat BOTH sides of the steel (22 gage) at the same time. I do not have problems with any of the following, curl, cracking, popping, orange peal etc. If you follow my instructions by cleaning the metal with my procedure, you should not have the problems that have been mentioned. Be sure to cover both sides of the metal with some sort of enamel prier to firing. My cleaning procedure allows for a perfect firing on both sides in one firing. I haven't had popping problems in years. Warping is caused by not having the piece properly supported.

That would be my understanding.

With a very stiff substrate and with an ideal cooling curve (irrespective of how it is achieved) how would enamel on one side "know" whether there was enamel on the other side? 

Of course we should remember that stiffness values should be taken at the elevated temperatures involved but stiffness will still depend on thickness.

Further: The way I understand things is you counter enamel simply to stop the substrate from bending.  Because that achieves a balance - what is happening to one side is happening on the other.

As far as the enamel is concerned, it does not know whether there is counter enameling on the other side or whether it's sat on a very stiff substrate.

So, when you counter enamel, it's akin to enameling on a stiff surface (logically), because the base stays put meaning the substrate does not accommodate itself to the stresses by bending. And that seems, in general, to pose no problems for copper.

But, if your substrate has a COE significantly different from the enamel, the glass may crack on cooling if the substrate is counter enameled or it's thick and unbend-able.

My understanding.

But insulation effects aside you do agree that given a sufficiently thick and sufficiently stiff metal one side cannot "know" what is happening on the other side?

At some point the thickness and stiffness of the metal substrate will dominate meaning that any mechanical effects of the enamel on one side are merely "skin" effects which do not transmit to sufficient depth in the material to affect the other side.

There are some interesting notes here.

http://www.iei-world.org/downloads/Articolo_faust.pdf

Why is everyone making this so difficult. I put ground coat on both sides, Why? because when the metal and enamel cools both sides pull together. To thick enamel will cause cooling discrepancies while cooling, don't use thick enamel.  By this I mean if the enamel is to thick you will get pinging, popping and cracking. The reason is that the enamel won't be able to stretch when the metal cools. To thick enamel will be to stiff for the movement of the metal due to cooling process. This will cause the popping, and cracking. The thinner you can apply the enamel the better, that's why I apply an extremely thin coat of ground. Once the ground is applied, the following coats of enamel can be applied as thin as you want, just be aware of you departures, and time. The will very with the thickness of the applied enamel. Are you using a mist or a heaver coat. I use a 22 gage cold rolled steel, and I don't have any problems with cracking, popping, pealing, or anything else. Once the ground coat is placed on the back side, there is no more need to apply any further coats of enamel to that surface.

>>Why is everyone making this so difficult

Charles - I dont think people are trying to make this difficult, people ask questions because they want to learn or understand what is going on, personally I do not regard that as a bad idea or a sign of a wish for complexity, indeed often the reverse.

Not everyone wants enamel on both sides. in my case I am experimenting with electroplating on the reverse side, I am also interested in other surface finishes for the "counter" side and so I am a "one-sider", other people may share my preference or be limited to the same decision for other reasons.

As someone with a good grounding in the technical background I do believe that there are some assertions in some of the artisan enamelling literature which may not stand up to scientific scrutiny and I do not think it hurts at all if people want to challenge existing ideas.

Its great that you have a method that works for you but it would not work for me and possibly others so please allow people to discuss these points as they see fit - this discussion is about enamelling, its on topic, one solution does not fit all I think it is a very relevent and worth while discussion.

I spent a long time doing my own research on this issue because I do want to enamel on one side only and I was interested in the differences in strength between steel and copper along the temperature range and also the the difference between steel and copper in how closely they match the COE of enamels over the entire temperature range.

In the end I turned to papers written by researchers working in the manufacturing industry precisely because I could not find detailed answers in the literature produced by the artistic community. By asking questions of myself and doing my own literature research I have resolved most of the issues I faced.

Hi Richard

I suspect that distortion and mechanical failure are very closely related for the following reason.

When you take a rod and bend it until it snaps you are putting the outer surface of the curve into tension and the inner surface into compression.

You are not placing the entire cross section into tension only the outer layer, the rest is in compression.

If you compare this with the amount of force required to simply pull the rod apart you will find a lot more effort is required to cause failiure than when you bend.

There is of course a hidden advantage when you bend the rod since you also benefit from leverage however the fact remains that shape distortion or bending allows you to place only a subset of the wood fibres running along the rod into tension. It is possible to cause mechanical failure with less overall applied tension.

This is not a simple matter of shape, an enamelled sphere is already curved, its about distortion, the shape of the composite at the point where the enamel had cooled sufficiently to exhibit strain compared with the final shape at room temperature. This together with how those distortions are reflected in whether the enamel is entirely in compression, entirely in tension of most likely a complex mix of the two.

Seen from the angle of a strategy to deliberately cause mechanical failure its about divide and conquer - dont try and snap all the fibres in one go - snap a subset and then move onto the next lot when they have failed.

The distortion or change in shape is a consequence of the forces involved and cannot in my view be considered in isolation.

If you enamel on thin sheet, the sheet will curl as the enamel cools. To avoid that you can counter-enamel on the other side. This proves then that you can enamel on a stiff base that won't curl and the enamel will not crack when it cools. True?