Bigger Crack After Using Amaco Bisque Fix

[Sara Kelley]

I recently made a coil pot and when it was fired it came out with hairline cracks. I put Amaco Bisque Fix on it and fired it again and the crack came out bigger than before and the Bisque Fix was flaking off. 
I was wondering if anyone knows how to fix the crack or why the bisque fix didn’t work. 

[neilestrick]

Any fix on bisque is a gamble. If it's applied to the surface it will probably flake off. In the crack it will probably hold if the crack doesn't get bigger, but it can't prevent a crack from growing. Whether or not a crack grows depends on what caused it in the first place. Post pics.

[Kelly in AK]

You fixed it after bisque, when you fired it again, what cone was that to?

The tension causing the crack doesn’t go away, it’s set up when you’re making the work. Firing exacerbates it. Bisque firing, a little bit, higher firing, a lot. A lot of tension makes a bigger crack. At cone 6 (or whatever your clay matures at) there are massive changes in chemistry and shrinkage. The two big shrinks are wet to bone dry and bisque to glaze, if you’re firing to vitrify the clay. Bisque shrinkage is negligible, but enough to expose hairlines that will greatly expand upon maturity in glaze firing.

Post photos and you’ll likely get a solution to avoiding the crack in the first place. You’re in a community of crack doctors. Or at least a crack perseverant population. 

[Sara Kelley]

[Sara Kelley]

Before I put the bisque fix on, there were only two little hairline cracks, not this big one. Also, thanks for the replies!!

[Sara Kelley]

It was bisque fired at Cone 04 and had a small hairline crack. We applied the bisque fix to cover the crack and fired again at cone 04 and the crack became enlarged, almost as if the bisque fix wedged it open. Now do I put bisque fix into the cavity again and fire or apply bisque fix to the crack and glaze on top and do a glaze fire at cone 05? 

[neilestrick]

That type of crack is likely to continue to get worse, especially in the glaze firing. Sometimes it's best to just move on and make another one.

[Pres]

@Sara KelleyFiring the pot the second time vitrified the clay body a little more, and caused more shrinkage of the clay thus enlarging the crack. When I have had issues with cracks of this sort in student pottery, I would have the student enlarge the crack by scraping out the area with a pointed tool like a can opener. Then clean it out well leaving no dust dampen the area and force the repair agent deep into the crack, and not have much over the clay surface around the crack by scraping the surface smooth. Fire to the normal temperature which in your case was 04. You have to realize the your clay body and your repair material probably have different shrinkage rates due to the firings - very difficult to repair after bisque.

 

best,

Pres

[Mark C.]

Some cracks never heal and some get bigger. Learning which are fixable takes experience . Yours is only going to get worse and if it where mine I would toss it and make a new one.I have used bisque fix for many successful repairs but knowing its limits will take time for anyone to learn

[Min]

Amaco doesn't actually say that their Bisque Fix works for cracks. From Amaco's website, "This white paste is an excellent mending material for bisque that can be used to bond pieces together or fill up broken cavities."

Different stresses within the claybody with cracks than reattaching a knob or filling in a depression/cavity. 

[Babs]

Also,if mending anything, the pot and mending paste have to be totally dry before refiring. Folk have had success, though not saying with yhis type of crack, dont know  , Spooze with paper added.