MamaJenXO

YEs I do like her things!
From what I have been gleaning in my search is that it is the glaze that will make the underglazes run. And that the Amaco Velvets will run with the correct glaze.
Sean O'Connell also fires at cone 6 and uses Jeff Campana's glaze receipe - available at Jeff's website - also check out Sean's things -beautiful also.
My problem is that I am limited to firing at cone 10 - so I am testing and asking for a runny-ish clear glaze for cone 10. 
So if you are firing at cone 6  - try Jeff's glaze - it should work for you - 
Also it really is just the blues that run - they have some cobalt in them - which is why she also adds blue to her blacks.
Thankyou so much for your input!
Jen

The bleeding is from the glaze!

I've got a girlfriend who has signed up for a Dawn Candy workshop, on the materials list she said she uses Amaco Velvets but can use other brands. Sean O'Connel's website has the glaze recipe he uses from 2018 as Campana Clear. I've tested that glaze, it is fluid.  (contains zinc so chrome green stains in green underglazes are going to go muddy)

I don't think adding flux to the underglaze is going to help much, unless you add enough that it actually melts like a glaze. Focus on the clear glaze. Needs to be fluid, and needs to be thick enough to move. I don't get any bleeding if the glaze is too thin.

I just had a cone 6 fire where I put Amaco undeglaze on the top of a glaze.....it did blend in better....it was on top of Amaco Snow Celadon. I tried the white on Amaco Mulberry celadon and it did not entirely melt. 

I follow a number of people on Instagram who use bleeding underglaze, and I've never seen them making it bleed in the raw stage. It's always the glaze causing the bleed. I can control the degree of bleeding on my work by simply making the glaze more or less fluid.

Amaco LUG black will run a bit, but not sure if that is what you are wanting?

Got to use a glaze which moves.
Guess you could use a flux in the underglaze..but note that or you'll use it sometime when you want it not to move.

Try some test tiles using the same glaze recipe but less of whatever type of clay is in it. I don't have the recipe for that glaze but if for example it has 25 epk in it then mix up the recipe but with 8 less epk then dip a test tile with the underglaze on it, then add 2 more epk and repeat, keep doing this until the total epk is 23 (for this example). The first test tile might craze with the epk reduced by approx 25% but there should be a tipping point where you get running from the glaze being more fluid yet not so overfluxed that it now crazes.

You need the clear glaze to move, and you have to put the underglaze thick enough that it'll get picked up. It shouldn't matter when you apply them. I do all my underglaze decorating on leather hard, and the glaze still moves it even though the underglaze gets bisque fired on.

i haven’t done this in a while but if i remember right the black mostly bled and i think a dark almost black blue bled.
does the clear move? i achieved the bleeding by applying the underglaze and immediately applying the clear glaze. lots of it - but not too much to cause the cloudiness.
thin lines have not bled that much. thicker lines with 3 or 4 coates of ug has bled. 
if i want the design to move i’ve applied glaze and then applied underglaze (though mostly cobalt) on top of the glaze. it does not bleed. it just shifts. 
also if i remember right, my walls bled, not the floors of the pots. 
good luck!!!