Post firing/glazing repairs

[Marla Stroup]

I have made a vessel sink for a guest bathroom. With this design there is no need for holes for the faucet and handles, the only plumbing accommodation is a drain hole so basically it is a large bowl with a hole in it.

Unfortunately, after the glaze firing (it had been previously bisque fired and looked fine) it developed a crack/split running from the drain hole into the body. I hate to toss out all the work that's gone into this piece and was wondering if any of you had experiences using epoxy type materials? I hesitate to re-fire it and don't know what this would involve anyway. The split is not totally vertical but runs on a slight angle. The glaze is a straight white so there is no color to match. Any suggestions would be most welcome. I was looking for some material that would be "workable" and have a finish similar to the glaze. Also I was considering "kintsugi" type of mending.

[Norm Stuart]

I haven't had a high success rate with repair firings. Cracks usually seem to get larger. But I've had more success with refiring a Cone 6 fired piece with Cone 06 glaze, white in your case.  At Cone 06 most cone 6 glazes get soft and blend flawlessly with the lower cone glaze.

I've also used two part epoxy with mason stains on bathroom floor tiles, as this is California and over time earthquakes do a number on tile floors.  Practice in advance so you can match the gloss or mattness of the surrounding ceramic.

Personally I'd try the Cone 06 fix.  You can even use the same glaze with an added 20% 3195 frit.  Test tile in advance is helpful.  I'd fill the crack on the left side and the one on the right side with grog before applying the glaze.

Sometimes it's better to rethink a piece and fill the cracks with a different color Cone 06 glaze and make the defects a "design feature".  I enjoy doing this with works other artists have abandoned after experiencing cracking and other problems. Finding a path to rehabilitating the piece back to the level of art.

[Vaughan L Smith]

I have made sinks for 15 years and this was my experience when I first made them. It never happens any more because I dry them very slowly, in a damp cupboard eliminating drafts,  cut them through with a wire, off the bat several times during the drying time. When throwing them I compress the clay back towards the center, pushing a thicker lump back in to the drain area, reversing the stress caused by opening the clay ball. This is very like the classic "s" crack problem, caused by lack of compression, easy to do when you are centering 25lbs of clay. If this is not a thrown piece it is probably thinner at the drain area and dried too quickly, or was stuck to the bat it was sitting on for too long. Firing them too fast in the kiln can also do this, I do an 18 hour firing, and raise the temperature from 1000F- 1150F by 69F an hour.

My first experience making sinks resulted in 11 out of 12 cracking. I have not had one crack in years following the above guidelines. As to your sink, sorry, it is now a planter, epoxy glue will plug up the hole but it will never be perfect. Your next one will be better if you use Laguna Bmix5.

[Marla Stroup]

Thanks for taking the time to help me out with this. My sink is hand built and probably dried over a 3-4 week period and it had come through the bisque firing without a hitch. I compressed the drain opening as best I could and I think that's what got it through the bisque. I will try making the next one with a thicker bottom and compress the opening again. I was puzzled and disappointed that it had made it through the first firing but split during the glaze firing. 

After initial drying do you think it would be better to move the piece to another "work board?" (I use pieces of sheetrock taped on the edges) or on an open wire rack?

[Marla Stroup]

I'm going to practice making just the bottom of the sink with the hole and see if I can get that to come through intact with no splitting. That way I won't have all the time invested in making a fancy sink only to have it crack during the last firing. 

I have a small roller that I'm going to use to compact/compress the clay in the area of the drain, do that, then cut the hole and compress the edges. Here goes...

[Vaughan L Smith]

Cut the hole when the clay is just past leather hard. Make sure the clay bottom is the same thickness as the wall and does not get pushed up by the bat or board it is drying on. Any pressure on the bottom forcing bending can create a crack later.

Marla Stroup said:

I'm going to practice making just the bottom of the sink with the hole and see if I can get that to come through intact with no splitting. That way I won't have all the time invested in making a fancy sink only to have it crack during the last firing. 

I have a small roller that I'm going to use to compact/compress the clay in the area of the drain, do that, then cut the hole and compress the edges. Here goes...

[Vaughan L Smith]

Is the clay stoneware or earthenware?

Marla Stroup said:

Thanks for taking the time to help me out with this. My sink is hand built and probably dried over a 3-4 week period and it had come through the bisque firing without a hitch. I compressed the drain opening as best I could and I think that's what got it through the bisque. I will try making the next one with a thicker bottom and compress the opening again. I was puzzled and disappointed that it had made it through the first firing but split during the glaze firing. 

After initial drying do you think it would be better to move the piece to another "work board?" (I use pieces of sheetrock taped on the edges) or on an open wire rack?

[Marla Stroup]

It is a porcelain stoneware mixture with no grog. It is called love stone from alligator clay out of Baton Rouge.

Vaughan L Smith said:

Is the clay stoneware or earthenware?

Marla Stroup said:

Thanks for taking the time to help me out with this. My sink is hand built and probably dried over a 3-4 week period and it had come through the bisque firing without a hitch. I compressed the drain opening as best I could and I think that's what got it through the bisque. I will try making the next one with a thicker bottom and compress the opening again. I was puzzled and disappointed that it had made it through the first firing but split during the glaze firing. 

After initial drying do you think it would be better to move the piece to another "work board?" (I use pieces of sheetrock taped on the edges) or on an open wire rack?