Underglaze Fading after Firing

[necropunk]

I've been spending more and more time trying out new glazes, but I recently came into an issue while making test tiles. I've working with amaco brand velvet underglazes, and made up a pile of test tiles for the colors I currently have. I used the same procedure with all of them (bisque the tiles, two layers of underglaze, dry, two layers of clear, but on some of the tiles the color seems to have just vanished. I had the issue once before with a cup I had underglazed before bisque firing, but there the color faded during the second firing. I've attached a pictures of the fading tiles for reference.

Has anyone had these issue before? Am I screwing up the firing, or is it an issue with the glazes themselves? For reference, I'm firing in the 04-05 range.

Thanks in advance.

[Min]

Hi necropunk and welcome to the forum.

Velvet underglazes (as all others except homemade ones using colouring oxides or carbonates) use stains to get the colours. Stains have certain conditions in which they work well or as you found out don't work as expected. It's a question of using a glaze that works with the underglazes. Are you making your own glazes or using a commercial one? It looks like some of the underglazes just went on too thinly. Did you dilute them too much with water? Top two look like you are getting the colour but it's just on too thinly. 

Amaco put out a field guide to their Velvet Underglazes being glazed and fired to low, mid and highfire. Link to that here, on the second page you can see they used Glazed with LG-10 Clear Transparent and fired to 05, it's a handy reference to see which colours fade or change with a higher firing.

[necropunk]

I'm using commercial glazes, I don't have the space or money to make my own at the moment. I'm actually using the same clear amaco used in their guide (thank you for that), LG-10 Clear Transparent, which makes me wonder if its an issue with the glaze itself. I haven't had to dilute any of the glazes I used for the test tiles, so maybe it is that I just need more coats.

[Min]

I'ld try a heavier application of the underglaze. Let each coat dry before putting on the next and brush each coat in the opposite direction to the last to get good even coverage.  Good to see you are trying them on test tiles, try a few with different glaze thicknesses and see what you get.

[Chilly]

I get newbies to do a "number of coats" test tile, before they do any glazing or underglazing.  A square of clay, marked into 4 smaller squares.  Corner 1 gets one coat of underglaze, corner 2 gets 2 coats, corner 3............etc

This gets bisque fired and then they apply 1 coat of clear on half of corner 1, 2 coats on half of corner 2..........

Second fire to earthenware, (we don't do any high-fire) then they (I) can determine if they are a light applier or a heavy applier.  This gives them a guide for future applications.    

We often repeat with a vertical tile with texture.

Edited July 6, 2022 by Chilly
typo

[necropunk]

Chilly, I have never considered that and will be wholeheartedly stealing the idea, thank you. I'm not sure about making tiles for every color all at once, but definitely for my next few colors I'll be trying that out. 

[Bill Kielb]

10 hours ago, necropunk said:

I'm not sure about making tiles for every color all at once, but definitely for my next few colors I'll be trying that out. 

Some folks make a color pallet painting a stripe of all their underglazes then covering 1/2 with their clear glaze and fire to cone so they have a reference with how each underglaze changes with a specific glaze applied over top at a specific cone. One big test tile, many colors, one, two, and three coats half covered with your favorite clear.

Amaco and Mayco also have samples already made and there are sites that have examples. One such: https://www.thestudiomanager.com/

 

Edited July 4, 2022 by Bill Kielb

[Callie Beller Diesel]

If you do them all in a batch, or at least all the ones you currently own, you have a reference for when you’re using them. As you acquire more colours, you can make the testers for them as you go.

[Chilly]

On 7/4/2022 at 4:53 AM, necropunk said:

I'm not sure about making tiles for every color all at once

I have samples of all the glazes I own, and we have quite a few at the centre, but my comment referred to application method/ability, not colour.  But I do get them to do another test before glazing anything they think is "precious".

Edited September 10, 2023 by Chilly

[necropunk]

On 7/10/2022 at 2:59 PM, Chilly said:

I have samples of all the glazes I own, and we have quite a few at the centre, but my comment referred to application bethod/ability, not colour.  But I do get them to do another test before glazing anything they think is "precious".

Ah, my mistake.