Colour density

[Mark Armstrong]

So I have done some colour batching since Petes class, copper and cobalt blues and numerous silver opal batches, all adhering fairly closely to recipes from that class. So I have a little experience, but not a lot.
I am wanting to do a simple copper blue batch. I have a piece of glass from a existing batch that I did. I love the colour of this piece, but would like to produce some pieces 4 times thicker. From the reading I've done is it safe to assume I would use 4 times less red copper oxide to produce a similar looking colour in the thicker piece.

[Pete VanderLaan]

Make a mold that is wedge shaped with the thickest end about 4 inches thick and the thin edge tapering to 3/4 inch. Ram cast that if you can and anneal it from the existing formula you have. It will show what the density of the color likely will be. Make it so the thick end is the top of the mold and the thinnest part at the bottom. That way you can really jam the glass deep into the mold. Wood is good.

Copper is peculiar stuff. It shifts expansions and viscosities substantially to the point of being problematic if combined with other glasses. Chuck used to call it an "Emetic glass". In my dictionary that suggests throwing up.

[Mark Armstrong]

Thanks Pete,
I had read about that. I was just wondering if there might be a direct correlation between thickness and oxide amount in some of the simpler coloured glasses.
Thankfully it is not being cased with anything.

[Pete VanderLaan]

copper turns sort of muddy when it gets really thick.

[Josh Bernbaum]

Sometimes Mark I find it can be helpful to just mix up 5 lbs. of batch, 10 at the most, with a certain amount of your colourant, let it melt flat and take a gather to see what the density is looking like. Then multiply to get the amount that would go in your full batch mix. It's not super accurate because I've found things tend to usually get a bit darker after fining, but can be a decent guide to get closer to the colouration you want without having to throw out a full melt that might not be to your liking. Also, yeah I second that copper blues can give you trouble fit-wise.

[Mark Armstrong]

Thanks for that little trick Josh, I will start on the light side and go from there.

[Pete VanderLaan]

If you are going to go that route, and it's the way to go, consider enough to make the wedge and then put 4 grams of red copper in the 20 lbs, no more. I think you'll be surprised at the intensity of the color on the thick end of the wedge. It will be very light on the thin edge.

But don't forget, copper gets really muddy and hard to read in thick sections. It also makes the glass really runny.
Keep the cullet. You can add it to something else later. You will never get a clean Cadmium yellow if it's ever put in that pot. Think mustard.