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In this last kiln, one of my projects was to correct a crawling fault in one of my prototype glazes.  In this post, I’ll talk about some of the approaches I used to attempt to correct the problem as well as my experiences with crawling in general.


A procedural note for the pottery un-initiated:  Dry, unfired ware is called “greenware”.  Greenware is bisque fired in a lower temperature kiln before it is glazed and is then fired in a “glost kiln”.

definition/causes:  During the melt (melting of the glaze), when a glaze separates from itself and reveals bare clay underneath, we call this crawling.  This is due to excessive shrinkage of the glaze from the raw state through to the final fired product.  The shrinkage is caused by the loss of water and/or gas (and therefore mass) and does not seem to be the same as shrinkage associated with the various oxides themselves, i.e. thermal expansion/contraction.  Several materials and processes can exacerbate loss of water or gas such as: binders, bentonite, clays, too thick a glaze coat, poor adhesion of glaze to ware (dirt/grease on bisque), glazes which are highly viscous when molten.  Loss of molecules in the form of water or gas is an issue because when those molecules leave, there is nothing to fill in the gaps that remain.  This makes little fissures which can lead to bare spots on the clay, particularly if the glaze does not melt vigorously enough to redistribute itself over the surface of the ware.  (see: (LOI – loss on ignition – http://en.wikipedia.org/wiki/Loss_on_ignition)      

In some highly controlled instances, crawling is a desirable effect and can be a very attractive art glaze but most of the time, it’s just a fault.  It reminds me of mange, but that’s just my opinion.  So, of course, I want to correct the crawling in this glaze as it is not highly controlled or attractive.

Environmentally Induced Crawling:

This spring we had a huge problem with crawling.  It started quite suddenly with glazes we had been using for several months.  We finally realized that it was due to the trees dumping pollen right outside the pottery door.  At that time, we didn’t cover our bisque ware and a fine coating of yellow pollen was on everything.  Almost all of our glazes at that time started to massively crawl right off the pots and onto the kiln shelves.  Getting glaze off kiln shelves is not easy, and you can imagine the language that was used.  After bathing the bisque ware with the garden hose and allowing them to dry, my crawling issue was resolved.    

We fixed this problem by building a clean cabinet of sorts out of a large plastic shelving unit, duct tape, and a roll of 4 mil plastic.  After pots come out of the bisque firing, Rob sands them (if needed), I number and wax them, and then they get a quick bath with the hose.  The ready to glaze pieces are immediately put into the clean cabinet where they can sit for as long as I need.

Intrinsic Crawling:

But sometimes, a glaze crawls and it doesn’t have anything to do with dirt.  In this case, it’s a fault in the glaze itself.  I have noticed that “fat” glazes are more likely to crawl than “thin” ones.  In Emily’s language, a fat glaze is one that holds a lot of water (associated water, not chemically bound water, likely has to do with deflocculation).  Fat glazes feel slimy and the glaze particles stay in suspension usually quite well.  When a bisque pot is dipped into a fat glaze, it will soak up a fairly heavy coat and take forever to dry.  This is a problem if I am in a hurry and want to put another coat of glaze on.  How many of my problems could be fixed by patience?  Fat Chance!    

A thin or skinny glaze tends to fall out of suspension generating a glaze particle “brick” at the bottom of the bucket.  They don’t hold much water, and bisque ware will not soak up much of these glazes.  I can apply multiple coats in rapid succession as the freshly glazed piece dries very quickly.  This is offset of course by the time it takes to dislodge the brick at the bottom of the bucket. 

In my experience, fat glazes are ones which have high proportions of Gerstley borate (a melting agent or “flux” and glass former), kaolin (a clay), magnesium (from various sources, dolomite seems to be more of a problem than talc), or some combination of the three.  Combos seem to cause more problems than any one of these ingredients individually, but that is just a general impression.  So, any time I want a vigorously melting (Gerstley borate) or satiny-matte (alumina and magnesium) glaze, I seem to have a problem.  It is important to note at this point that merely adding water to a fat glaze to thin it does not seem to work – in my experience.  That is, in every coat fewer glaze particles are deposited onto the surface of the bisque.  This makes for a very puny glaze.  So, I have to add more coats.  I seem to end up pushing the glaze particles farther apart and end up saturating the bisque.  Crawling still ensues.  Ah, physical chemistry.

My current problem glaze has both kaolin (Edgar Plastic Kaolin, EPK – http://en.wikipedia.org/wiki/Kaolinite) as well as dolomite (http://en.wikipedia.org/wiki/Dolomite), both at 10% of the total weight.  I had read that ball clay was supposed to aid crawling.  So, I did a 10% line blend covering all the combinations of EPK and OM4 (Kentucky Old Mine 4 – ball clay general: http://en.wikipedia.org/wiki/Ball_clay) in 1g increments.  Indeed, the raw glaze containing OM4 was thinner than the EPK-containing version and permitted a thinner glaze coat.  However, all of the mixes in my line blend crawled to some extent.  The OM4 10% crawled the least, but <grumble grumble sigh>…

In addition to the line blend between EPK and OM4, I also tried some other unique recipes.  Fortunately, Robert had the brilliant idea of reducing the LOI of the glaze.  As I mentioned above, water and gas are released from the melting glaze and this can cause problems if they are in excess.  I also mentioned that dolomite seemed to be more of a problem than talc.  We (finally) started using our science brains and worked out that dolomite, calcium magnesium carbonate, evolves carbon dioxide gas when heated.  Thus, replacing the dolomite with another magnesium containing mineral, talc, might help.  The improved recipes were derived by inputting my initial recipe into glaze calculation software (Thank You Insight! – http://digitalfire.com/index.html) and replacing the “fat” dolomite with “skinny” talc (http://en.wikipedia.org/wiki/Talc).  I also ended up adding wollastonite (http://en.wikipedia.org/wiki/Wollastonite) to maintain the calcium level and decreasing the flint to maintain the silica.  These alterations significantly decreased the calculated LOI of the recipe.  The resulting glaze was just as nice as the original and didn’t crawl in the initial test. WooHoo!

The Plan:

I have made a medium sized batch of this new recipe and will test the beegeezus out of it in the kiln I am putting on tomorrow – thick/thin/single/double/triple dip.  Robert must throw more test pots (more bad language, from Robert this time).  I’ll also be testing the higher calcium version of the clear glaze.  Calcium bleaches the iron in the clay body creating a clearer clear.  It was nice in the prototype, but I don’t assume one test pot is enough to bank on anymore.  Let’s see…  I am also trying the Tenmoku with black iron oxide instead of red, and I am still trying to get the blue black to work (see Facebook “lapped black” photos – http://www.facebook.com/ejwoodward?ref=profile#/photo.php?pid=2296653&id=88705631526).