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Amaco velvet underglaze appearing strangely from glaze firing!


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Hi there, 

I am trying to make some plates for my sisters wedding present and I have been running into a lot of problems! I am using Amaco velvet underglaze (fire to cone 5/6) on greenware - when they were fired for bisque (I put the underglaze on the greenware before the bisque) they came out ok (although still quite dark) but then i fired them for glazing (with transparent on top for glazing part as per instruction and to get glossy plate look) and  they have come out very strangely - its almost as if some of the glaze has disappeared somewhere and it looks significantly thinner that it was after the bisque firing.

Here is what I used:
Clay: White Special Stoneware (1200-1300)

Bisque fire:

I am wondering what the reccomended bisque for this would be in degrees celcius eg. room temp - 600*C at 150*C per hour (I have sadly forgotten which bisque i had)?

Glaze fire:

I think I will try the following schedule:
- room to 600 at 100 per hour

- 600 to 931 at 150 per hour

- 931c - 1031c at 60 p/h

Do you think this would be ok?

I would love to know if anyone had any reccomended glaze and firing scheudles for this clay and the amaco underglaze as I am keen to get the plates right as theyre for my sisters wedding!

I am not sure if the problem is in my firing ramp or if because of the hire fire clay we have (sadly I dont have any others at the moment)
image.png.c5c5897de56180f7fd8b8f5a70cc4fb9.png
THANKY YOU!

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5 hours ago, larathompson288 said:

...and  they have come out very strangely - its almost as if some of the glaze has disappeared somewhere and it looks significantly thinner that it was after the bisque firing

@larathompson288, from your post here, not trying to repeat myself but it really is a good idea to do some test tiles as not all glazes are compatible with all underglaze colours. Try out 1, 2 and 3 coats of underglaze on test tiles and see how they look. How much are you watering down the underglazes or are you using them as is?

Also, the clay you are using White Special Stoneware (1200-1300) needs to be fired to the upper end of its range to make durable ware.  I'm not clear when reading your post, are you firing them to 1031C as the top temperature? Reason I ask is that by firing it only to 1031C (if that is the case) it will be quite under fired. This can have ramifications on its durability plus porosity. Porosity is important, especially if the pots are not glazed all over. Moisture will get into the claybody causing pots getting very hot in the microwave, weeping pots and by under firing to such a large amount the glazes will probably craze with the moisture absorption over time.

 

Edited by Min
grammar
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In addition to the glaze maybe not being compatible, underglazes generally need at least 2-3 coats to show up well. If you're applying to bone dry ware you can probably get away with 2, but if you're applying to leather hard, you will need 3-4 coats to get good coverage, depending on the color.

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Just to add, it might be the picture but the overglaze appears maybe not fully melted. Again, it might be the picture or how that glaze typically melts. I mention because it’s generally considered good practice for wares in contact with food to be reasonably durable.

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