Min

Gums will help with the adhesion of the glaze layers if that is the cause of the crawling but there are other things to look at with crawling glaze problems too. Overall thickness of the glaze layers and also how well the top glaze is bonded with the base glaze are things I would be looking at here too. If there are cracks or lifting of the dry glaze then chances are it will crawl when fired. If you add gum try for the least amount you can to solve the problem. I'ld suggest mixing up a very small test batch of 100 or 200 grams of base plus water plus 1/2% of CMC or Veegum Cer. Try it with the gum in just one of the glazes first then layer the other glaze over it. Dip a few test tiles for the same length of time you dip your pots. (test tiles should be same thickness as your pots and bisque fired the same) Now add some gum to the second glaze (again just a small test batch) and dip some more test tiles. Get the second coat of glaze onto the test tiles as soon as the wet sheen is gone and you can handle the pots. Don't wait for the base glaze to dry all the way. Glazes with gum will take longer to dry so using the least you can is a good idea. If the 1/2% isn't enough then double it and try again, it might take up to 2% gum. Decreasing the water content of the glaze sometimes helps with thin walled pots too. Since you are using commercial glazes we can't look at the recipes for those and adjust clay content etc. If you can get a product called Magma in the UK then I would try that, if not then try either CMC or Veegum Cer.

If you are thinking there will be 5-6% rutile in all the glazes (expect chrome / tin red) then I would add that to the base glaze for all points with your blends. That way you won't have to use one axial of the bi, tri or quad axial for the rutile, it will give you the full amount of rutile in each glaze test. So for the blue / green trial I would do a triaxial and use copper carb, cobalt carb and red iron oxide for the 3 points. If you don't already have one I would get a syringe with cc's marked on it to measure out the amounts. If the base glaze you are testing is either of the recipes I sent you a couple weeks ago I wouldn't bother testing it with chrome/tin. There isn't enough calcium in them to support this type of red. (plus the alumina is probably too high)

I believe B-Mix with grog is going to be much finer than 35 mesh. I'm guessing it's more like an 80 mesh.

Have you tried just using the B-Mix with grog that you already have? I've used that for raku with good success.

Sounds about right, depends on how quickly you are heating and cooling the pots etc. Fine or medium grog? If you guesstimate the wet clay having around 20% water you will be a bit over your 20% grog target but it should be close enough. Have you tried cut and slam wedging to get the grog mixed in? Video below if you need it. When I've added grog (or sand) to clay I make a rough block shape with the clay, slice it up and sprinkle the grog on each layer then slam them together. Add water with a spray bottle as you need. Once the grog is layered into the block then do the slice and slam wedging to get it mixed in. https://youtu.be/HApNjUnI9U4

@Aminah Bradford, could you post a picture of the bubbles? If it's a pyroplastic clay and there is even a tiny air bubble it can bloat, especially if the clay is over fired or on refires.

Really sorry to hear this Pres. Best wishes for a speedy recovery when she gets her surgery.

@tzylawy, I'm copying the image you posted in the gallery into this thread so it's easier to find. Definitely wouldn't fire this as is. Was the glaze frozen at some point or is it really old? Also did you really stir it up before using? Did your brush drag as you were applying the glaze? Reason I'm asking is it looks like the gums that are typically used in brushing glazes are no longer effective. Is that the same glaze on the inside of the pot? I'ld scrape off as much glaze as you can (wear a mask), dump it back into the jar and scrub the outside of the pot. I'ld then add a little bit of gum solution and enough water to make a good brushing consistency and try it again on a test tile. If it works on the test tile then reglaze your pot. (it's easy to make your own gum solution if you don't want to buy it) Welcome to the forum

Okay, so the clear glaze doesn't fit the clay. For commercial glazes it's really a matter of trial and error to get one that fits. I'ld start with contacting the supplier of the bisque and asking which clear glaze they recommend. If you use any green underglazes it's best to use a zinc free clear. Zinc can mess up any underglaze colours that contain chrome and turn it an ugly brown. Are you using cones to verify the glaze is getting to the correct cone? Underfiring can cause a glaze to craze too.

Haven't used Sheffield or Bailey claybodies so I'm no help making a recommendation. Hopefully someone will chime in here with their recommendations. I do have a couple pieces of Bailey equipment (mixer/pugger and wheel), I've found their customer support is really good so I'm a bit surprised to hear about their clay having issues with bloating. Did they offer any help with the issue?

@ashhorth, are these commercially made mugs that come glazed then you do the decorative work on? Could you give a bit more information on your process?

Hi Bill, The point I was trying to make was if one wanted to test for an underglaze being too refractory or overly fluxed adding a material to increase or decrease the melt would be a simple test. Again, it's lovely that the glazes you use work. Given that altering glazes is out of the reach of some potters adding Gerstley Borate (or a boron frit like you mentioned some members in the studio you are member of do) is an easy way to do increase the melt of an underglaze. Adding kaolin to to do the opposite. 2 materials only, a measuring spoon and no scale needed. A very simple old underglaze recipe is simply kaolin, Gerstley (or frit), stain plus water. Not the best recipe but it demonstrates the core materials. Yes, thank you, I am aware of the oxides that make up Gerstley Borate and EPK. Just as we refer to potassium or sodium feldspars doesn't preclude the fact that both contain other oxides also; I assumed it was a given that we are supplying more than just a pure flux oxide or alumina oxide. Adding Gerstley Borate will increase the melt, adding kaolin the opposite. If my suggestion of using 1 tablespoon of underglaze seems to be wasteful then by all means scale it down.

Re the debate of a refractory underglaze or not it would be fairly simple to test this theory without altering glaze recipes. Since we know adding flux will make things less refractory and adding alumina the opposite, using this logic I'ld run 2 parallel tests, one adding flux, Gerstley Borate should be a good choice, and the other adding calcined kaolin. Say roughly 1 Tablespoon liquid underglaze + 3/4 teaspoon of Gerstley Borate or calcined kaolin plus enough additional water to make them brushable. Apply to both a vertical and horizontal surface with the same number of coats and fire them both the same and see what comes out of the kiln.

Lovely that you found a solution that works for the potters in your studio but from reading the original post from @carolrossit sounds like she is using a commercial brushing glaze. I think we need to work within the parameters of the the op's usage insofar as commercial versus studio mixed glaze. Like many things in ceramics there are times we can make things more complicated than they need to be.

A potash feldspar? What boron frits do you have? edit: have you tried other iron saturates without boron?