Copper dies are the gold standard for foil stamp dies. Copper has the greatest retention of heat of any metal. Heat and pressure are the most significant factors in foil stamping. It holds fine line type very well because the metal is very hard and it etches close to straight up and down. The metal also expands very little when heated to high temperatures. The only real downside to copper is it’s cost.
While magnesium foil dies is an inferior product when compared to copper, the emboss dies may be a better product. The most important factor is that magnesium bevels can be altered in the etch much easier than copper. Remember that copper etches straight up and down. This is great for foiling but not good for embossing. The straight edges tend to die cut when embossing. With magnesium, we can create bevels that are much gentler when embossing paper. This single factor is why I think magnesium dies may be a better choice on emboss dies. The fact that magnesium is softer and does not hold heat as evenly does not matter as much while embossing. We are a now pouring, pre-cast counters for our magnesium dies. If you have never tried a magnesium emboss die, give it a try. I think you will be impressed!