Substituting Aluminua Oxide For Aluminua Hydrate In Kiln Wash

[vivajones]

Can i substitute aluminua oxide for aluminua hydrate in making a kilnwash with calcined kaolin or EPK?

[Mark C.]

I have no idea why you want to. I do not have a good sense one way or the other-you could test a small amount?

Hydrated alumina works so well why bother substituting?

Mark

[Guest JBaymore]

Expensive kilnwash unless you are doing soda, salt, or wood firings. Alumina compounds of all types are pricey compared to kaolin and flint. Yes if you have Alumina oxide on hand and are out of alumina hydrate you can sub it. The issue will be the ratio of alumina in the wash will be slightly different due to the different molecular weights between the oxide form and the hydrate form..... but with kiln wash this is a moot point. And that is assuming you are making it up by weights... not volumes.

 

A very typical wash is 50% kaolin and 50% flint. For extra shelf protecting "firepower"..... 1/3 each of kaolin, flint, and alumina.

 

best,

 

.......................john

[perkolator]

my gas kiln wash is 90 alumina  , some kaolin, macaloid, and CMC -- yes, it's quite pricey wash, but this is the wash our studio has been using for decades in the gas kilns and has proven to be quite bulletproof against student disasters - so why change?  if I were paying for the materials myself, I'd definitely be using a less costly wash.

 

i've used both oxide and hydrate.  you can substitute one for the other in a kiln wash since molecular weight is not as important here as it is in a glaze, but they will act slightly different in the bucket -- the hydrated version stays in suspension much better.

[Guest JBaymore]

Considering the fact that Laguna Clay / Axner sell low-quality high-silica kiln wash for $1.70 per pound - I simply don't understand why alumina has such an expensive reputation.

 

Well....... here in the NE from my supplier (Sheffield) alumina oxide sells for $66.00 for 50 pounds, while alumina hydrate sells for $38.00 for 50 pounds.  EPK is at $20.00 for 50 pounds, and flint (Silcosil 200) is at $13.50 for 50 pounds.

 

So a 50% EPK and 50% flint wash (which DOES do the job unless you are firing salt, soda, or wood) comes out to about $0.335 per pound.  A 50% alumina oxide and 50% EPK wash would cost $0.86 per pound.  That is a significant price difference over time.  With alumina hydrate used instead, that cost comes down a bit to $0.58 per pound.  But still almost a buck difference for every two pounds used there too.

 

Even for general wood firings..... a general shelf wash with over 33% alumina hydrate is "overkill" in my opinion.  And I've been firing wood since 1969.  My current wash is 33.3% each alumina hydrate, flint (200m), and EPK.  Some of my SiC shelves for the noborigama are over 40 years old now (kinda' big and heavy, unfortunately.......... love my Advancers).  That wash works.

 

Even in college ceramic department settings (studet work and abuse) I've regularly used 50/50 EPK / flint.   

 

Why ANYONE would spend $1.70 per pound on kiln wash once they know enough about ceramics to have bought a kiln to fire is pretty a total waste of money.  Weighing accuracy for kiln wash is NOT all that important, and even mixing by VOLUME 50 / 50 with any kaolin and any flint will give you a "servicable" shelf wash.

 

best,

 

.....................john