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What Causes The Stuff At The Top Of The Bucket?


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calling all chemists!    

 

some of the glazes i use do something strange up at the top.  i usually keep the buckets cleaner but these are ones i used last november and left until may 1 or so.  i have seen this happen in the past but it is not something i worried about then and i am not really worried now, just curious about what ingredient is causing it.

 

one is the rutile green in bill vangilder's book and the other is called sapphire from ?  30 years ago???

 

Rutile green (this is colored with mason stain cerulean blue, not green)         

                                                                                                                         

whiting                     4

talc                           5

EPK                        17

custer                     22

silica                       26

frit 3134                  26

--------------------------------

                              100

add rutile                  6

       bentonite           2

 

 

Sapphire  

 

frit 3134                 42.6

custer                    25.5

EPK                       25.5

silica                      15

-----------------------------------

                             108.6

add  rutile                4.3

        cobalt oxide     2.1  i prefer to use cobalt carb but i do not know how to convert the numbers for this size.  i usually mix up a 

                                       2000 gram batch at a time.  this is a beautiful glaze but covers texture so i only use it on exteriors of bowls

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Not really sure: but the whiting, silica, and EPK would not be causing it. Not enough talc, so magnesium is more than likely out. So that would only leave soluble salts from the custer (most likely), or the less likely boron from the 3134. Soluble salts can cause hydrolysis if they sit for awhile: hydrolysis just means the molecules separate. Looks like it is separating to me.

 

Nerd

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Separates molecules: sodium chloride would separate into sodium and chloride for instance. The chunks as you call them are more than likely sodium crystallized; along with whatever is attached to them. Technically it means positive and negatively charged particles separate in the presence of water.. Separates in chemistry is called disassociation; in marriage it is called a divorce.. sorry- chemistry humor. 

 

Nerd

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is the separated part missed by the whole recipe?  if it falls off into the batch and i blend it in will it return to its original state?  is removing the flakes entirely changing the recipe?  should i change the custer to ?  

 

the forum clock is an hour earlier than it is here and i want to go to bed.  zzzzzzzzzzzzzyou tomorrow.

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it does flake off.  i will try that just for you, nerd, i haven't touched either of them after i used up all the glaze.

 

i remember one bucket years ago that wound up having very large "potato chip" flakes all over the top of the glaze.

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This is what sodium does to glaze after it sits for an extended period. Mr. Baymore was commenting on recycling poorly formulated porcelain; and it acting like wet beach sand ( I believe was his description) Anyone want to venture a guess what the primary flux used in those poorly formulate porcelain bodies?  Sodium based glazes and clays are best used in a short period after mixing/getting them: sodium has long term reactions that cause problems later on.

Nerd

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Would like to see someone just take a sponge and wipe through the crusty: then let it sit. I would expect to see it flake off because I strongly suspect it is sodium crystals.

Let me try this again:

 

 

Would like to see someone just take a sponge full of vinegar and wipe through the crusty: then let it sit. I would expect to see it flake off because I strongly suspect it is sodium crystals.

Nerd

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The gunk on one of my glazes buckets doesn't dissolve in vinegar. Glaze is this recipe plus stain. I scraped some of it into a pot, covered in vinegar, left it sit for a couple hours or so then sieved it. Want me to leave it sit in the vinegar for longer? (I can get it off buckets with drywall sanding mesh and elbow grease) I swiped at the stuff with straight vinegar, loose crumbly bits flaked off (outer edge of crust) but rest is still firmly attached to the bucket.

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Here is the exact chemistry of what is occurring in your glaze bucket:

 

 As water evaporates from a salt solution, salt ions are no longer surrounded by solvent molecules. Therefore the ions will interact with each other, and due to the strong forces resulting from their opposite charge will reform a crystal lattice. The solvent in this case is water; and sodium is hydrophobic; meaning it ejects water. As the crystal forms an ionic bond, it ejects even molecular water making it harder than a rock- because it is a rock. Stronger acid would do the job, but I am not going to suggest that for safety reasons.

 

Let me nose around a bit and see if there is some organic solvent you can add that prevents the sodium crystals from forming- or at least inhibit the growth.

 

Nerd

 

An interesting side note: sodium crystals actually increase in size and density in the presence of halite. Halite is a naturally occurring salt that is commonly found in clay/s. The salt from the Nep Sy, Minspar, or other feldspar bonds with halite creating dense sodium crystals.  Halite actually accelerates the growth of ionic crystals. So those recipes with higher percentages of clay will cause even more problems than those without clay.

 

Additional note: like any crystal, the presence of metallic oxides will color them. Sodium crystals favor iron and manganese, but will absorb any metallic oxide into the crystal lattice.

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Potassium bitartrate is used as a buffer in chemistry: meaning it is used to buffer (hinder) a build up of alkalinity caused by the disassociation of sodium ions. It is also used to prevent crystallization of sodium products, mostly do to the fact that potassium bitartrate produces tartaric acid as it disassociates.

 

So where do you find potassium bitartrate?  It is better known as "cream of tartar", in the baking isle at your grocery store. I would start with one teaspoon per quart of glaze.

 

Nerd

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thanks, nerd.  i assumed the sponge would have vinegar in it.  no time yet so should i bother based on Min's post?  

 

cream of tartar seems to come only in tiny spice bottles that are expensive for their size, is there something else we can ask for at a clay supplier?

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(What Causes The Stuff At The Top Of The Bucket?)

The stuff in the bucket is what causes that stuff-every glaze is different-some leave hard residues some leave none.Some are soft.

I never worry to much about it. I do not clean it unless its a problem and that is very rare.When it interferes with the lid closing and I'm mixing a new batch up I may work on the residue .I'm not a clean freak . 

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Tartaric acid (cream of tartar) is available on amazon or ebay for $9 a pound. Tartaric acid is the chemical name for potassium bitartrate. Commonly used as an anti caking agent, crystal inhibiter, and a few other food purposes.

Nerd

 

Note: I have a gut feeling this or something similar is being used in commercial glazes to prevent this problem. Never hear of this problem from commercial glaze users.

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thanks again, nerd.  this has been very helpful to several members, i think.  that $9 for an entire pound is too good to be true when it is sold in tiny bottles for over $4.  my bottle holds 42 grams!  someone is making a profit.

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I've noticed this a bit over the years too. Glad to know what the cause is, and I'll be looking over the recipes of the buckets I find with the crust on them.

 

There is a place in Atlanta on east ponce called the dekalb farmer's market. They sell spices and a lot of other things in bulk for dirt cheap. I imagine they also get stuff like cream of tartar in bulk and would order cheap if one asked them to. There's probably some sort of restaurant supply/wholesaler in most big cities that would order a quantity of cream of tartar for you if you know the right people to ask and how to ask them. Sometimes bringing along a nice mug or cup as a gift gets folks in the mood to help.

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