Clear glaze washing out rich dark brown clay

[tkw954]

I'm working with a wild clay that fires to a really nice rich, dark brown color when fired to cone 6 electric. It's quite vitrified at this point and will slump so I've been making some marble pieces with a commercial clay that works out nicely.  However, when I try to apply a cone 6 clear glaze ("Mastering Cone 6 Glazes" glossy clear liner") the dark browns turn to light tan. Is it possible to formulate a clear glaze that won't wash out the  clay's deep color?

I've attached a picture with the unglazed marble on the left and the glazed version on the right. My marbling process changed a bit so only look at the color of the dark parts, not the fineness of the marble. You can also see the effect on the rim of the left cup as I applied a liner glaze with a rim around the outside of the lip.

Edited May 20 by tkw954

[tkw954]

I should add that I'm slip casting, so I'd be willing to make additives to the clay if it would help preserve the color.

[Min]

The only way around this is to add 2-4% red iron oxide to the clear base to tint it amber which will look more like the dark clay. Problem with this is it will also appear amber on your white lay and in some glazes the iron  produces specks in some base glazes. 

Part of the problem is from the calcium in the glaze, it’s in most, try avoid high calcium glazes. When I looked up the MC6Glazes liner it is quite high in calcium at 0.64 molar.

Edited May 21 by Min

[Callie Beller Diesel]

As someone who chased this particular dragon: no. I played with formulas and firing cycles, and you’re always left with at least some micro bubble clouding. 

Min’s right about the best way to make a glaze look clear over a dark clay body being an amber. If you want to play with marbling, you could try using a buff clay instead of the white. 

[Rae Reich]

You might consider using other glazes besides clear that act/appear differently over the two different clays. I made some pieces of porcelain and dark red stoneware that I glazed with a rutile ‘pink’ (applied thinly because it gets runny) that went dark blue on the stoneware and rosy on the porcelain. Celadons will give you contrasts, too. Experiment! Test!

Edited May 22 by Rae Reich