An oya, or olla, is a terracotta vessel that’s buried in the ground to water the garden. Not only does their use dramatically reduce water waste but also watering time. So, buried in the ground next to your courgettes, you’re bound to get a bumper crop with very little fuss!

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oya in the ground

(Photo: S Matthews, Shutterstock)

 

Before you begin:
We are using a plaster mould here to make the body of the oya. We’ve used a globe-shaped mould, but you could use any symmetrical shape that will allow for two halves to be joined together – it is simply a receptacle for water, so shape really is irrelevant. It won’t be seen because it will be buried up to its neck underground. If you need to make a mould, it will take a couple of weeks to dry out, so you’ll need to do this before you’re ready to make the oya.

It should be noted that these vessels can be made in any size – including miniature versions to keep your houseplants watered.

 

 

Making the plaster globe mould

A basic globe-shaped mould is probably one of the most useful the potter can have because it can be used in so many ways. From slip casting to press moulding, you will be able to produce items as diverse as light shades, vases, kitchenware or garden ornaments to name but a few, and all you need to make the mould is a cheap plastic ball.
These instructions are for a mould that was used for slip casting,  so ignore the fact that some of the photos show a ‘spare’ (this is a space for pouring liquid slip into the mould, and isn’t needed here).

 

You will need:

• A plastic beach ball – available in several sizes, look for ones that don’t have textured surfaces (like footballs)
• Potter’s plaster – the amount will depend on the size of your ball
• Scales for weighing plaster
• Measuring jug
• Large buckets for mixing plaster
• Plaster clay – (clay reserved exclusively for mould making – remember, once it’s been used for plaster work, clay can’t then be used for any other purpose)
• Flexible material to make a cottle – lino or more clay
• Mould maker’s size (soft soap)
• Newspaper

Before you begin:
Prepare your workspace so that everything is to hand and all other materials (like clay) are well out of the way, to avoid plaster contamination.
Line a bowl with newspaper, to collect debris

 

ball to make plaster mould

 

 

 

Tie a length of string around the centre of the ball to establish a centre line.

 

 2

Following the line of the string, carefully draw in the centre line with a marker pen.
Remove the string when finished.

 

3

Measure the height to the central line and make small adjustments until it’s equal all the way around the ball.

 

Begin to build up a clay wall clay around the ball to completely contain it to the central line.
Aim to extend the wall 25-35mm out from the ball.
You’ll find this task easier and the outcome far more accurate if you build up the side of the ball in small stages.

5

Refine the level of the clay at the centre line by filling in, where necessary, with thin coils of soft clay.
Use a flat-sided rib to smooth over the surface once the level is correct to the line.

6

Roughly smooth and level the sides of the clay wall to vertical, so that the cottle will fit flush to it when it’s fixed in place. (Ignore the fact that this photo shows a ‘spare’)

7

Position your lino, or clay cottle around the model, (Note, it must extend at least 10cm above the top of the ball) and secure it with tape at the base, middle and top.

TIP: masking tape is perfectly acceptable for this job providing you wind it around the lino several times.

8

Run a thin coil of soft clay around the inner edge of the cottle to seal the space between the clay wall and lino.
Smooth the coil in carefully with a finger – it may be a little tricky to get your hand into the space, so take care.

 

9

Seal around the outside base of the cottle with a thick coil of soft clay – it’s important to ensure there are no possible escape routes for the liquid plaster when working in this way – everything must be very secure!

 

10

Working on the recipe of 2 pints water to 3lb of plaster, estimate the number of pints it will take to cover the model in the cottle including a 10mm excess at the top.

The measurement will vary according to the size of ball used to make the model so specifics can’t be given, but for the model cast here, 7 pints of water were used – it is quite a large ball!

Measure the water and transfer it to your bucket.

Weigh out the correct amount of plaster and sprinkle it into the water until it’s all absorbed.

Stir the plaster with your hand (WARNING – wear a rubber glove if you have any allergies).

 

11

A chemical reaction in the plaster will cause it to heat up as it sets. Once this has happened, it’s safe to remove the cottle.

 

12

If the top of the plaster is uneven, level it off with the side of a metal ruler, as shown. Surform the edges to remove sharp burrs which would otherwise break off and possibly contaminate your clay when using the mould.

 

13

You now need to turn the whole thing over to remove the clay from around the model without pulling it out of the plaster. If you find this difficult because the clay has sealed onto the work surface, simply underwire it, as shown, to release it before turning it over.

Remove the clay carefully to avoid disturbing the model and the cork.

Store the clay in a clearly marked bag – CLAY FOR PLASTER – and don’t use it for anything else.

 

14

Wipe away all traces of clay from the ball and surface of the plaster.

 

15

Shave away any sharp areas around the edge of the plaster with a surform before continuing. Remove the ball and wipe any clay out of the mould with a damp sponge. Put the mould somewhere warm to dry out. Try the airing cupboard, or on top of a range cooker like an Aga if you have one. Be aware; it will take several weeks to dry out thoroughly – you will have to be patient.

 

To make the oya

You will need:

• Red earthenware clay
• Plaster mould
• Rolling pin, roller guides, plastic sheet
• Small texturing tool. See the wonderful range MKM tools has to offer – you will want them all!
• Foam blocks to support the shape in construction and drying

 

1

Have your mould close at hand, ready for use. Make sure it is clean and dust-free.

 

 2

Prepare a block of clay large enough to fit inside the mould once rolled – you’ll have to estimate the size according to your mould.

Working on the sheet of plastic, reduce the bulk of the clay by beating it with the side of your rolling pin. Work in measured, even strokes from one side of the clay to the other, to avoid making deep grooves in the surface.

Roll out the clay between your roller guides.

You will find that periodically turning the slab will make rolling much easier and give you more control to achieve the approximate shape you need.

Once rolled out, smooth over the surface of the slab with a rib to compact the clay.

 

 3

Cut a semi-circle out of the slab, which is large enough to fill half of the mould.

 

 4

Gently ease the slab section into the mould using a barely damp sponge to avoid marking the clay with your fingers.

 

5

Cut out a second semi-circle and place it in the mould so that it overlaps the first half by a couple of centimetres – again using the sponge to ease it into place.

 

6
blending clay into oya mould

Blend the overlapping edges together thoroughly with your thumb to ensure the seal has not trapped air between the layers.

 

7

Cut away the bulk of excess clay above the rim of the mould, making sure the knife doesn’t come into contact with the plaster.

 

8

Work over the interior surface of the join with a rib, removing excess clay as required until the walls are evenly thick.

 

9

Now remove the remainder of the clay at the rim with the side of a batten resting on the mould.

Work in small steps, drawing the batten sideways and backwards to remove small sections at a time until the rim is level.

 

10
unmoulding ball for oya mould

When the clay has firmed enough for the shape to hold, turn it out of the mould onto a board.

 

11

Reinforce the overlap join on the outside of the shape with a coil of soft clay.

Carefully press the coil across the surface into the join with your thumb.

 

12

Work over the reinforcement with a rib, removing excess where required for a neat finish.

 

13

Work over the reinforced area again with a soft kidney until all evidence of the join is eliminated.

 

14

Make the second half of the globe in exactly the same way.

 

15

Sit one half of the globe on a foam block with a hole cut out at the centre to support it, then score the rim with a serrated kidney.

Repeat the process for the second half.

 

16

Apply slip to the scored rims, then fit the two halves together carefully.

 

17

Reinforce the join with another coil of soft clay. Blend the coil in well using your thumb or finger, then smooth over the area with a rib, again removing excess clay for a neat finish.

 

18

We don’t often say this, but you can change tools if you think you have one that is more suited to the job at hand. We all have our favourites, and every maker will use several types to form an object.

Bearing this in mind then, feel free to change the rib for a kidney to achieve a better finish.

 

19

The shape and finish of the oya don’t really matter because, after all, it will be buried underground. However, for the personal satisfaction of knowing you have made something to the best of your ability, it is worth aiming for as good a surface finish as possible. So, for example, if the shape is a little distorted and you really would like it to be round, try paddling the surface with a wooden spatula to improve it.

If you would prefer the globe to have a base to stand on (this makes firing much easier if you’re nervous about placing unusually shaped items in the kiln), simply tap it on the work surface a couple of times to flatten the underside until it will support itself.

 

20

Look for a circular cutter that’s large enough to get your hand through.

Place the cutter on the top surface of the globe and score the outline with a pin.

 

21

Carefully cut out the circle with a sharp knife and lift it off the globe.

 

22

This is where it is important to be able to get your hand inside the globe. Reinforce the join on the inside with another coil of soft clay. Blend it in well, then remove the excess as much as possible using a rib.

 

23

Roll a thick log of clay about 12cm long, then gently push a length of 10mm dowel through the centre and out the opposite end.

 

24

Holding the dowel at each end, roll the log on the work surface to enlarge the hole at the centre. Continue until you are just able to get your fingers inside the tube.

 

25

With the tube sitting on a whirler, now begin to pinch the wall to extend the size of the tube. Begin at the midpoint and pinch outward and upward in small rhythmical movements to ensure the wall is an even thickness.

 

26

Turn the tube over and pinch the second half in the same way until it is level with the lower half. The wall should be even throughout, and the wall vertical when finished.

 

27

Test the size of the tube on the globe to make sure it is wide enough to fit the opening.

 

28

Return the tube to the whirler, then turning it slowly while holding a knife rigid in your hand, carefully cut the rim level.

 

29

Now refine the outer surface of the tube using a metal palette rib to ensure it is vertical. Remove clay as required for a smooth surface finish.

 

30

Score around the opening of the globe, then the rim of the neck. Apply slip to both surfaces.

 

31
adding oya spout

Fit the neck onto the globe and reinforce around the join with a slightly thicker than usual coil of soft clay.

Blend the coil in with your thumb.

 

32

Work around the join with the round end of a rib until smooth and tidy.

 

33

NOTE: – this step isn’t shown, but if you’re unhappy with the size and shape of the neck, you can change it by cutting out four V-shaped sections from the rim (opposite to one another) then joining them back together by overlapping the cuts and blending the clay together thoroughly. This is really only possible if the clay is still relatively malleable, but you can use slip for extra security if you feel it is needed.

Blend over the cut joins and the join at the shoulder with a long throwing rib to fully seal them together.

 

34

Similarly, if you feel the neck wall is too thick, you can pare it back with a surform until correct.

Once you have removed the required amount of clay, smooth over the surface with a rib or kidney to remove the marks left behind by the surform.

 

35

The basic form is now complete, and all that remains to do is make a lid for it to prevent the water from evaporating too quickly when the oya is in use.

 

36

Form another short log-shaped coil about 3cm long and push the dowel through the centre as you did earlier.

Roll the log to open out the centre until you can get your fingers inside.

 

37

Place the tube on a whirler and pinch the shape until wide enough to fit inside the neck of the oya with a little wiggle room.

 

38

Cut the rim of the tube level as you did the neck, then quickly check that the size is correct again and has not been distorted in the cutting.

 

 39

Working on the plastic sheet and using 10mm roller guides, roll a small slab of clay.

 

40

Now using your chosen texturing tool, impress the surface of the slab. This example uses an MKM roller HR 052 – Dragonfly Party design – very apt for the garden.

 

41

Measure the size of the lid you will need to cover the top of the oya. Luckily, the cookie-cutter shown was just the right size, but you could make a card template if you don’t have a cutter in a suitable size.

TIP A multi-sized selection of cookie cutters is really invaluable in pottery. They are inexpensive to buy, so well worth investing in a set – you’ll wonder how you managed without them!

 

42

Cut out the lid from the textured slab, carefully choosing the best area to cut it from.

 

43

Turn the lid upside down and centre the tube on the surface. Mark the position with a pin.

 

44

Score the rim of the tube and marked position on the lid, then apply slip to both surfaces and secure the two together.

 

45

Neaten around the outside edge of the join with a wooden tool to remove excess slip, then reinforce the inside of the join with a coil of soft clay. Blend the coil in seamlessly with a wooden tool.

 

46

Check the fit of the lid in the vessel – it should sit neatly on the rim of the oya with a little wiggle room to allow for shrinkage in firing.

Allow the vessel to dry with the lid in place, then bisque fire as usual.

 

A note about firing:

Because this oya has been made as a globe, it doesn’t have a base to sit on in the kiln. You can resolve the issue of keeping it upright by standing it on a suitably sized star pin or support it around the base with kiln props – either method will work.

Alternatively – make the oya with a base – it will work the same way!

 

47

Once bisque fired, a good way of keeping the oya upright until ready to use in the garden is to sit it in a bucket, as shown.

 

48

The great thing about an oya is that it only needs to be bisque fired. When low fired, the porosity of the clay is the key working element of this vessel – allowing water to seep through the pores slowly to keep the ground moist.

(Photo: S Matthews, Shutterstock)