I tend to think that once I know something about ceramics, it is a thing everyone knows and is obvious. My own knowledge and experience seem routine to me, so it's always a surprise when I make a comment about a process and the people I'm talking to don't know about it. This happened quite recently with mention of paper clay patches, so I'm going to explain it here. If you already know about it and this is all obvious, you can skip this.
Clay shrinks as it dries, and it shrinks during firing. Sometimes if you are joining two bits of clay and they are at different stages in the drying process, they will pull apart as drying continues. I started using paper clay slip for joining parts because of this. The paper in the clay tempers the different shrinkage and lets the bond take place successfully. So that's a good thing, but that's not what I want to talk about. Paper clay can also be used to fill cracks and mend breaks at the greenware or bisque stages.
I'm using mugs as an example here because I happen to be making mugs, and I had some sprung handles. It's been really hard to keep the moisture content right with the heavy rains we've been having. My studio is very rustic and influenced by the weather. The timing was off with a few of these handles, so this is what happened. I love to make mugs, and I want each mug to be special to the person who uses it, so I don't mind slaking down failed mugs and trying again, but for the purpose of illustration, I'm going to show you some repairs. If you have some work you've been labouring over for months on end and it gets a crack or break, you might want to try this technique to save it rather than melt it down.
Here we see a sprung handle. The handle was a bit too dry when I put it on, and although it seemed to show enough flexibility at the time, that turned out to be just one of the sneaky tricks porcelain plays. It was too late, and when I returned the next day, the handle had cracked, and the mugs were well past leatherhard, approaching bone dry. 
This clay is Valentine Clays Royale porcelain. I assume it should go without saying, but use the same paper clay as the original clay body for this sort of thing.
In the mean time, I noticed a small crack in a pot that had just come out of the previous bisque firing. This is an occasional side effect of the way I alter my pots. Again, I might have just thrown this out, but I was thinking of you and your need to know, so went ahead with the patch. It's made of a different clay, Valentine Clays V9G. I didn't have any V9G paper clay on hand, so I made some up.
First I ripped up some old egg boxes. You can use any paper for this. Lots of people swear by toilet paper, and I've heard of people using computer paper or newspaper, or whatever is around, but I like egg boxes because I learned that they are an end of life recycled product. Paper can be recycled up to seven time (I have no idea how the tell how many times it has been recycled, tell me if you know), and when it's done, it gets made into egg boxes. Egg boxes cannot be further recycled, so they go into compost. Compost is a good thing, but I think it can spare me a few egg boxes. Here I have used the whole lid and half of the bottom of a 12-egg box. Don't worry about the colour, it burns out.
Add water and pulverize. It's fast. It doesn't have to soak for ages, just go ahead right away.
Next I glopped in a bunch of the squidgy V9G from the bucket of wet clay waiting to be reclaimed, then pulverized it together with the paper so it's nice and smooth and consistent throughout the batch.
Some people find a benefit in being precise in their measurements. I'm precise in glaze mixing, but I don't find it matters for this. I've heard of paper additions being anything from an iota to about 50%.
And it's good to go. Be sure to label both lid and container if you're using more than one type of clay in your studio.
Next, I work the paper clay into the crack. It could subside into the crack a little, so let it dry for a short time and then cover it with a little more. Neatness does count here because this is going to get glazed without another bisque firing. I have, in the past, reattached little broken off bits with this paper clay on bisque technique, and it's been fine. It might not always be fine, but if your piece has become un-fine without trying this, you have nothing to lose and possibly something to gain.
Back to porcelain. Bisque firing done, mug ready for further processing. I can still sort of see the line where the gap was.
I do generally go over my mugs with a sanding pad before glazing, just to make sure no little stray sharp bits are lurking on the underside of the handle or on the rim where lips might object to them. I smooth off my signature on the bottom so it doesn't stab people who hold the mug from the bottom. Reconstructive surgery takes a little more work than usual, but not vastly more. If the piece is worth it, you won't even notice the extra minutes.
I use silicon carbide embedded griddle pads, made for cleaning flat grills in restaurants. Sandpaper would do just as well, and is probably easier to find. I also use a metal tool to ping the easy bumps off. The handle repair is just going to take a little extra rubbing with that pad. Bisqueware sands easily, paperclay bisque doubly so. It's sort of like firm chalk.
Once done, it looks much like any of my handles. The repair is not obvious unless you're really looking for it. Maybe not even then.
An overnight stay in the kiln with a trip to 1300 degrees C, a day to cool, and out they come.
Done.
Let me know what you think!
Thanks for the info, I have worked with clay since 1992 and wish I had heard about this years ago!!
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