Tips for Firing Pottery Successfully

By: Northcote Pottery Supplies

Planning your Piece

Mark the bottom of your work with your name and date and clay type. The ideal thickness for handbuilt pottery is 1 – 1 ½ cm. Radical variations in the thickness of pottery promotes uneven heating and therefore cracking.

 

Very thick pottery is hard to dry fully and therefore will not fire successfully (i.e. it cracks, explodes) due to trapped moisture in the pottery heating and expanding.

 

Drying your Work

The a) more joins, b) larger the piece or c) more uneven or variable the thickness of a piece, the more chance there is of cracking occurring in drying.

 

Drying should be slowed by putting pieces into a plastic bag and taking up to 2 weeks to dry slowly & evenly.

 

Never rapid dry clay because it has a memory for the stresses which are created in making and these, coupled with uneven drying, can cause major disasters.

 

Never expose drying pottery to extreme cold or hot or variable conditions (i.e. leave near windows in winter or draughty doorways in hot summer wind).

 

Firing

Always know your clay type and glaze type - is it earthenware, mid-fire or stoneware/porcelain? The clay bag or glaze bottle will tell you.

Never put glaze on the bottom of pots; 1/2 cm up from the base is the best cut off line for glaze application - this ensures that there is no chance of excess glaze running and sticking your work to the kiln shelves.

 

Always read the instructions on glazes and underglazes and follow the manufacturer's recommendations. Do not over apply!

 

Transporting Pottery

Newspaper is not always a good wrapping product for transporting pottery because it can often damage fragile work.

 

Pizza boxes are very useful for transporting slabs/plates.

 

Stockings filled with sawdust can be useful packing, as can straw, shredded newspaper etc.

 

Never pack layer upon layer of pottery into a box unless the largest and heaviest work is at the bottom and lightest at the top.

 

Remember, pottery items may move in transit! What will the consequences be for the pottery forms if they knock into each other?

 

Keep a note of what pieces you bring for firing so you can be sure to collect all pieces when firing is complete.

 

Mending raw clay with vinegar based slip rahter than water based slip

This technique is only suitable for leather hard and almost dry pieces which are cracking, or have fallen apart. A vinegar based slip will help rejoining, but will not act like a glue to hold weighty pieces in position. Vinegar based slip gives much better results because vinegar is a flocculent which improves the possibility of a good mend and retards re-cracking. Once you have used the vinegar based slip to execute a mend, return the piece to a plastic bag and dry very slowly. Vinegar based slip is simply a mixture of the same clay that the piece is made from (in dry form), combined with any cheap vinegar to make a thick paste. Vinegar based slip could easily be used as your standard joining slip at all times. It's a good way to recycle any spare dry clay. Keep a pot on each work bench in the classroom.

 

Paperclay slip for raw and bisque joins/mends

Commercial paperclay bodies can be wet down to a thick paste to create a joining and mending slip. It is best if this slip is the same type as the clay to be joined (i.e. terracotta paperclay for joining terracotta clay). If you don't have the appropriate paperclay body, you can use dry fibre fluff ($15 for a large bag) which is the equivalent of the paper in paperclay, to create your own matching paperclay slip based on the clay you use. Simply stir some fibre fluff into your usual wet joining slip.

 

Mending raw/bisqued clay with fibre cement

Remember: it's a 'big ask' to expect a mending product to stick a major weighty piece of clay onto another piece of ceramic work. Be reasonable in your expectations. Use the fibre cement as a glue to hold the two pieces together in position. Sometimes it's helpful to use masking tape to hold the pieces in place until they are fully dry. Once fired, the fibre cement changes from mint green to white, and you can easily glaze over it to disguise the join.

 

Mending fired pottery items

The Goddess Aralditus of Adhesion is really the best solution for these repairs, in particular, the five minute (2 part) Araldite. There really isn't any other glue that is as effective. Super Glue is impossible! Clear silicone bathroom sealer is acceptable for joins where much of the clay work is missing or chipped off and the glue needs to be a filler as well as a glue. But, silicone is very difficult to get where you want it and keep it neat at the same time. Tarzan Grip is a very ordinary glue other than for sticking little pieces onto small works.

 

Other joining/mending techniques

Some potters swear by using a paste of water-based clay slip with sodium silicate (a deflocculant material) for repairing raw clay. However, this technique often causes a crispy, flaky glossy area that is far too visible and not very nice to glaze over.

 

Sometimes repairing is not worth it

Don't be afraid to start again! Some repairs are so traumatic and take so long that the whole piece could have been remade in the time it takes to repair. Try thinking outside the square and consider whether the work can be mended using alternative methods - wire, cane, plastic tubing, nuts and bolts etc. can all be utilized in a more innovative and integrated way to become part of the finished work.