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<p>[QUOTE=&quot;GDJMSP, post: 15919, member: 112&quot;]I agree with your comment that the gas bubble is already there when the planchet is formed. But the pressure at that time is not great enough to produce a bulge in the clad layer. Thus the planchet is flat when the dies strike it and the coin is fully struck. But when the coin is struck - this produces a lot of heat. The heat causes the gas to expand. And during striking some areas of the clad layer are thinned by the metal flow and thickened in other areas.</p><p><br /></p><p>Now combine these two things happening at the same moment - the gas expanding - the heated, softer metal thinning - and the gas now has the abilty to cause a bulge or bubble in the clad layer. Once the gas cools - it contracts again. But by then the metal has already been stretched and the bubble formed. Thus the bubble remains on the surface of the coin.</p><p><br /></p><p>All of this happens in the blink of an eye - less time even. I hope this helps explain the process.[/QUOTE]</p><p><br /></p>

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