This post is in response to queries on how to throw large items similar to the water filters.
Begin with a suitable size, 4 - 5 kgs. It is easy to work and stands up well with a 10-12 mm wall. This means thrown sections will need joining to create wanted size.
The joint I use is a ‘V’ joint shown in (fig1). The tool I made is from an old credit card (you could use the current one) shaped as shown. (fig2)
With the ball of clay at a size of your choice throw the base and wall to height as far as possible with a 10 - 12 mm wall. Throw this on a batt so you can remove it from the wheel.
You will need a source of heat, electric heat gun or better still a gas blowtorch. These are available at your hardware shop.
With the blowtorch dry your thrown piece to a point where you are confident it will hold the weight of the next section. Trim the top off level and form an upside-down V. = /\ with your new tool. (fig3)
Take this part including batt off the wheel and begin again on another batt.
This time throw a cylinder with 10 - 12 mm wall thickness.
Using calipers make the top of your cylinder the same diameter as the first section.
Use tool (fig 2) to form ‘V’ in the top edge and use heat gun stiffen top half of the cylinder .
Adjust diameter as there will be some shrinkage due to drying slightly from stiffening.
Measure the diameter at the top point of the first section; and bottom of the ‘V’ on the second section. (fig4)
SECTION THREE (the tricky bit)
Remove the current cylinder, including batt, from the wheel and replace with the first section.
Brush slip on top of the inverted ‘V.’
Using a third batt turn over section two. Take hold of the batt (bottom original batt is now on top) and place the cylinder on top of the base. They should fit if you measured correctly.
Cut the batt off the top cylinder using cutting wire.
Blend the join by using two straight wooden ribs. (fig5)
Now pull up the rest of the clay to thickness 10 - 12 mm.
Level top of the cylinder and start again on the next section as done in section 2.
Depending on what shape you want, this is the time to begin shaping sections one and two. When satisfied with the shape use the heat gun and stiffen the bottom section. (fig6)
Continue with sections until you reach your wanted size.
This month’s gallery feature is a yellow double wall bowl made with my raku mixture of stoneware and raku clay and wheel-thrown in sections before joining using slip. After a bisque fire, I apply the coloured glazes with the aid of masking tape and fire to 1080 degrees. The final fire is in the raku bin.
Here is a photo of a commission completed for Nautilus Restaurant in Port Douglas. Check them out if you dine there. I believe Nautilus has an exceptional degustation menu which we intend to try. (As a special treat for the secretary, bookkeeper, marketer, web page designer, blogger, typist, salesperson, debt collector, accountant, IT specialist, photographer, tax agent, file clerk, systems analyst, purchasing officer and cook.)
Use clean shelves
Shelves with glaze spills need cleaning as the glaze can eat into the shelf. Grind the glaze flat to the shelf using a flat diamond wheel on an angle grinder. It is not the perfect solution but you can get away with it by using a couple of coats of kiln wash on the shelf.
There are different views on the number of props to use. I use 3, but some prefer four. Most of my shelves are more than 30 years old and as new after some 500 plus fires using three props. All props should be in line one above the another. Staggering the props can cause bent shelves.
It is not ideal to use three and four prop setups in the same fire.
I occasionally use four props for large items when the centre prop is in the way. I use a four prop method once only and on the top of the load - so minimal weight is transferred away from the three prop tower. With the four prop method, you need to check for shelf rocking on three props. Therefore, the 4th prop will require packing. Pack with fibre paper for small variances or if shelves are too wonky – use clay wads.
When I’m repairing kilns, I see many full-sized shelves in use. First, this is hard on your body and as they are heavy require two people to place in the kiln. Half shelves are a better option and make packing the kiln much easier.
Select items of similar heights and load the half shelves on three props up the back of the kiln first. Then load the other half at the front. Try to stagger shelves to create good airflow through the kiln.
NB shelves 400 x 400 and 450 x 450 can stay full. Shelves above 530 x 530 should be cut in half.
Packing the kiln
Again views differ on how tight to pack the kiln. Some like a good gap between items. I pack my bisque tight to the point the things are touching and place small pieces inside large pieces. Obviously, we do not do this in a glaze fire; however, items are placed close together. While packing, keep in mind your cones and the shelf required at peephole level.
Starting the kiln.
Later model electric kilns are more straightforward as you can now select the program, press start and almost go to bed. In a bisque fire, initially, leave the bungs out. Around 350°C – 400°C I put the roof bungs in. If possible, put the door bungs in at 650°C – 700°C or do as I do and leave the door bung until the following morning when the firing has finished, and the temperature is still above 700°C.
Glaze firing in an electrically controlled kiln can have all the bungs in by 400 deg as by then the moisture has gone.
Unless you have an automatic gas kiln – manual support is necessary for the whole firing. Temperature control and burner control is every half hour with the use of a manual pyrometer and cones at the end of the fire.
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