Expert: Sam Kelly - 8/13/2008

Question
QUESTION: Hi Sam,
I appreciate if you give some advice about my problem.
Can we glaze a glazed ceramics after fired? I mean one of my friend has a big
project and she does a panels with ceramics. Before she checked the glaze
with small piece of a ceramic and there wasn't any problem existed. After she
did on a big panel and the glaze's color has totally changed.
Now we're thinking to glaze the glazed panels again, because she doesn't
have time to go back and do the panels again' so we think that it's good to
ask to an expert that possible know what we will get.
I don't know maybe it's not a good choice to glaze the glazed ceramics again,
or what we can do?
thanks,
Sumeyye Unver.

ANSWER: Hi Sumeyye,
Test the glaze again on a small piece to make sure the glaze is correct, if you used the same glaze to test as was used on the bigger pieces(both from the same glaze bucket) then we can assume the test and used glaze were the same.
The panels can be re-glazed to a point, they are no longer absorbent so what you do is warm them up, not to hot, but hot to touch and apply the glaze. The heat will evaporate the water from the glaze leaving a thin layer of dried glaze on the panel, to thick and it will crack and fall off.
Success isn't guaranteed but this is the way i have seen it done, even using a microwave oven to heat the pieces.
What comes to my mind is applying the glaze again with a brush(at room temperature). It's worth a try to add a brushing medium to the glaze so it turns into an underglaze consistency. 'brushing medium' is sold by pottery suppliers for potters to add to decorating colors making them more of a paint like consistency so they can be applied easier to dry bisque ware. It's relatively cheap here in Australia, $9-50 for half a litre. Mix up only the amount you will need and do a test, 2 parts glaze to one part brushing medium.
Take some time to look at the panels as they are, is the color really ugly or is it acceptable for thier purpose(does the exact color really matter).


---------- FOLLOW-UP ----------

QUESTION: It looks like a good result. Thanks Sam.
I talked with my friends about your email, and she wants to ask another
question to you. Does the "brushing medium" (we've never hear like this
liquid, but we'll search)possible to cover a yellow ferro glaze?
My friend told that she used 9kg (I hope you use Europe metric system) white
glaze(shiny) and just use 20gr. yellow ferro on it.  After to fired, in the result
the panels all color is salmon.  
Thanks for your time,
Sumeyye

Answer
The brushing medium can be added to any thing, all it does is make the underglaze or glaze in your case much easier to brush onto a surface.
As am example if you were to be trying to paint on something with milk it would be a difficult process, add one third brushing medium to the milk and it would become more like cream, much easier as it flows better.
I have done a search on FERRO for you and have uploaded a page of thier catalog for you, under application you will see hand brushing.

http://www.samkellystudio.com.au/allexperts/brushingmedium.pdf

Do you mean your friend painted 20g of a yellow ferro product onto/over the shiney glaze?

The test piece has to be the same clay, glaze, ferro color and fired to the same temperature. I am thinking that somewhere between the test piece and the panel firing something has changed. Go back and evaluate the process, there is no other way, look at what I have mentioned, did we use the same clay?, was the color out of the same bottle or did we use another?, was the kiln fired to the same temperature as the test?

If you are looking for a yellow and you are going to paint ontop of a darker color I do not think you are going to have success.

Sam