Pit fire, glaze and mugs

[Maisiekennett]

Heya, 

I dont know much about pottery and ceramics

I want to make some clay mugs by hand and fire them in a pit fire. Any tips for a pit fire? And will i be able to glaze the mugs so that they are waterproof? Even if i just glaze the inside and lip. 

Thanks,

Maisie

[liambesaw]

Best option for pit firing is to bisque pots first, but I don't think anything pit fired will be functional. And no, I don't think there's any glaze that can melt at a low enough temperature to use in pit firing.  Clay and glaze need very high temperature.  But you should try and see what happens, only way to learn!

[shawnhar]

Would there be a problem with bisque firing first, then doing the pit fire for effect, and after dipping in clear and firing to full temp?

[Min]

18 minutes ago, shawnhar said:

Would there be a problem with bisque firing first, then doing the pit fire for effect, and after dipping in clear and firing to full temp?

The carbon from the pit fire will totally burn out in a glaze firing, even an 04 firing is too hot.

[Benzine]

Yeah, sadly effects from Pit, Raku or other alternative forms of firing are not really achievable with functional wares.

If you use said method, to get the effects, the wares are no safe to use (Besides the glazes or carbon coming off with use, the clay body will be very porous and absorb/ seep liquids).

If you try and fire them hot enough, to make them functional, all the effects will be undone, by the higher firing.

Lastly if you tried to bisque a clay body to maturation, then pit or Raku fire it, the ware wouldn't be able to handle the thermal shock. 

 

However, it might be possible, with a little practice, to use mason stains/ oxides/ underglazes to create "fake" smoke/ carbon effects, then put clear glaze over those.  A simple black iron oxide stain wiped off, gives the look of age/ soot to pretty much any clay body.

 

[Lbegley]

On 3/29/2019 at 1:38 PM, Benzine said:

Lastly if you tried to bisque a clay body to maturation, then pit or Raku fire it, the ware wouldn't be able to handle the thermal shock. 

 

I wonder if you could use a low expansion clay body and get away with firing it to maturity first, without the thermal shock being a problem?  A friend of mine who does a lot of pit firing told me that the lower the bisque temp, the more color and smoke the ware takes on,  so a fully vitrified piece may not be very pretty even if it did survive. 

[neilestrick]

1 hour ago, Lbegley said:

I wonder if you could use a low expansion clay body and get away with firing it to maturity first, without the thermal shock being a problem?  A friend of mine who does a lot of pit firing told me that the lower the bisque temp, the more color and smoke the ware takes on,  so a fully vitrified piece may not be very pretty even if it did survive. 

You could use a flameware body to survive the pit, but it won't take any color since it's not porous.

People have been trying to find a way around this for decades, and no one has figured it out yet.

[glazenerd]

Neil:

if flame and oven ware are done in the 4-4.50COE range- what range do you think would work?

T

[neilestrick]

9 hours ago, glazenerd said:

Neil:

if flame and oven ware are done in the 4-4.50COE range- what range do you think would work?

T

They're two different animals, so the COE range doesn't matter. The pit requires porosity for it to work.

[glazenerd]

Neil: got the porosity issue , did not know if there was a preferred COE.. Apparently not.

[neilestrick]

1 hour ago, glazenerd said:

Neil: got the porosity issue , did not know if there was a preferred COE.. Apparently not.

A question for you, because my experience with COE is limited to making a glaze fit: Does the COE come into play when the body is really underfired like in a pit? I've seen pit work done with everything from porcelain to super groggy raku bodies, but is the fact that it's so underfired downplay the need for specific COE?

[glazenerd]

Low fire bodies typically run between 7-9.50COE, unless talc has been added to lower that number. Vitrification does not begin until 2050F. A cone 6-10 porcelain body fired lower would have a COE of 10 or above: the higher the silica %, the higher the expansion when under fired.

correct, the COE of pit fire would be high regardless of body formulation. Again, the higher the silica%, the higher the expansion. The reason pyrophyllte is used in raku, or other "thermal shock" bodies; much more tolerant of rapid changes in heat.

silica starts expanding at 1064F, (563C) 

[Magnolia Mud Research]

On 3/30/2019 at 5:01 PM, Lbegley said:

I wonder if you could use a low expansion clay body and get away with firing it to maturity first, without the thermal shock being a problem?  A friend of mine who does a lot of pit firing told me that the lower the bisque temp, the more color and smoke the ware takes on,  so a fully vitrified piece may not be very pretty even if it did survive. 

several comments relevant to the Lbegley's idea of pit firing effects on 'functional' ware:

Clay bodies with non-porous interior surfaces and porous exterior surfaces appear to be  impossible within the conventional clay box. 

1. The solution to this conundrum requires the potter to think outside the standard clay body box.  

2. my observations: 
I have been successfully using low fire (cone 05) clay bodies as coatings on clay bodies (stoneware and porcelain) fired to cone 10 reduction.  Similarly kaolin and other refractory clays, including the sandy clay from my ponds, applied to the surface of cone 10 stoneware and porcelain clay bodies produce semi-porous surfaces on the fired items.  

Long ago, I used an Armadillo Clay commercial low fire white clay body (cone 05) and fired the clay to cone 3 in oxidation; the fired ware was strong, had '0' measured water absorption, and did not slump or bloat. (the red version of the clay body performed the same). 

3.  Gedankenexperiments for pit fired (and other techniques) ware:

Gedankenexperiment A:  

Consider a vase say 10 cm diameter and 20-30 cm in height using a commercial white stoneware or porcelain that is non-porous in the cone 3-6 range.  While still on the wheel, coat the surface with a stiff slip of a cone 10+ clay body (more paste than slip) to 3-4 mm thickness.  When the sheen on the applied layer is gone, compress the surface with a rib and then set the vase aside to dry.  Fire the vase to maturity for the substrate low-mid-range cone clay body with the interior coated with a liner glaze.  after the firing the interior surface should be water tight and the exterior should be porous for several mm and suitable for pit firing and/or Raku techniques.  

Gedankenexperiment B: 

There are several so called 'Raku' clay bodies that require cone 10 or higher to have low water absorption.  Throw the vase with the such a 'Raku' clay body, line the vase with the low fire clay as a slip, then apply low fire liner glaze, bisque fire the vase to cone 3,    The interior should be fully functional and the exterior surface porous. 

Have fun with your pit firing.

 LT