Expert: Sam Kelly - 7/31/2008

Question
QUESTION: I have no experience with pottery, but I have some items purchased from Greece that I would like to have glazed for occasional food and drink use.  These are clay pieces.  The seller could not provide any information to me on how they were made.  What would be the best course of action, if any, to get these glazed?  TIA.

ANSWER: Hi John, the major problem here is the firing temperature of the glaze. Clay is made with different raw materials to fire to different temperatures, thus requiring a particular glaze to fit that clay body to make it food safe. The biggest problem with food safety is crazing of the glaze, tiny fine cracks appearing in the  glaze that will eventually harbor whatever was placed in or on the pot as well as absorption of water when washing.

The pots you have will have to be at the bisque fire stage, fired to a stone like consistency with no glaze application , to test this touch the tip of your tongue on the raw bisque pot, your tongue should feel like it wants to stick to the pot. The reason for the sticking feeling is to tell you if a wet glaze will adhere to the pot, in other words absorb onto and stick it's self on when dry. If there is no sensation to the tip of your tongue then the glaze will not stick to it and firing will not be successful.

Any sort of body oils from handling or dirt particles will also make glaze application a disaster. If you have determined that the pot will take a glaze and they have been handled a lot a refire to 700degC would be advisable.

Chances are that the pots are Earthenware, maturity for firing at earthenware is 1080 to 1120degC.
Stoneware is 1240 to 1260degC.

Select one of your pots, determin its friendliness to glaze application, apply an earthenware clear or light colored glaze and have it fired. Because you have no experience with clay find a professional potter or studio and have them do it for you. Make sure the glaze is a tested, tried and proven problem free earthenware glaze, the reason being is just because its an earthenware glaze it will not necessarily like your earthenware body.

If everything looks fine after firing wait 2 weeks and scribble a permanent marker(on the bottom) on the glaze and immediately rub off, if there is crazing it will go into the cracks. If it crazes there will be 2 scenarios, The glaze did not like your clay body or it is a stoneware clay.

NOTE: If you decide to fire to stoneware and the clay is earthenware it will melt and ruin the potters kiln shelf.

Should your pots not be used for food consumption then some light crazing will not matter, the crazing process is used by potters as a decorative effect and is acceptable as such.

Sam




---------- FOLLOW-UP ----------

QUESTION: Thank you VERY much Sam.  My tongue does indeed stick, so I suspect that a glaze will adhere based on your description.  

Would the refire cause any problem to the applied ink that is already on the pot?

Also, if it is fired to an Earthenware temperature (and I also believe it to be earthenware), would there be any chance of it melting if it were something else?

The piece is the first one on the following page, upper-right.  It's a bowl-shaped object on a foot with handles:

http://hellenic-art.com/pottery/bla5.htm

I would be glazing the inside and the lip for drinking.  The other piece is a plate with a small lip on it, and again the inside would be glazed for cakes and such.

Thanks for all of your help!

John

ANSWER: Hi John, at a guess i would say the refire to 700degC would not effect the applied ink as a glaze firing is much higher.

An email to the supplier is advised as they state "quote"The vases and the other pottery items are handmade either of clay or ceramin. All the items are painted by hand and signed by the artisan who created them"endquote".

NOTE:I have no idea what ceramin is, at what temp does it fire to?.

--------------------------------------------------
I have had a look at the website you supplied and th FAQ says.

The vases and the other pottery items are handmade either of clay or ceramin. All the items are painted by hand and signed by the artisan who created them.

All the pottery items can become fully functional and compatible with food once they are glazed in the inside. Unfortunately we do not glaze pottery items on request any more. You can have the item glazed yourself at a local ceramic workshop. Once glazed, you can use the lagini for oil, the hydria for water, the kylix and the kantharos for wine etc, as long of course as they are thoroughly cleaned before the first use. They are not suitable however for dish washer
-----------------------------------------------------

Sam

---------- FOLLOW-UP ----------

QUESTION: Sam,

I saw that FAQ and sent a number of e-mail inquiries to the supplier.  The supplier was unable to furnish the information that I requested.  I believe that their FAQ is probably true, but they have no idea of what the actual requirements are.  

Hopefully a glaze firing will also not affect the ink.  Do you think the firing temperature for glaze will cause a problem?  Also, it sounds like the earthenware is the lowest temperature for firing for a glaze.  If so, it sounds like the danger of melting is low.  

I'm going to assume that it is earthenware and plan accordingly.  Again, thank you for all of your help!

Answer
They say that thier pots can be fired so it's probably an underglaze that was applied to the dry clay before the first(bisque) firing, if so everything will be ok.
Bisque is usually 1000degC and earthenware glaze firing is around 1100degC, only 100deg more, i would say your pretty safe.
If they have not been any help leads me to the suspicion they import pots from a supplier elsewhere, not manufacturing themselves, is there any indication on your pots by way of makers marks that may suggest this.

Sam