Large scale ‘site’ clay press mould.

 

CHAPTER THREE

LARGE-SCALE PRESS MOULDING ON SITE

Press moulding can be an economical and surprisingly accurate method of mould production. Clay press moulding is probably the most fundamental method of mouldmaking. That is, it encompasses the most basic of mouldmaking principles, taking a ‘negative from a positive’, by the most fundamental of methods: using wet clay to produce a mould from an original. This is mouldmaking at its most basic, the use of wet clay pressed onto the surface of an original or the pressing of a pattern into the clay to create an impression (the mould) and then casting directly into that clay.

The use of clay as a moulding material can be considered as similar to the synthetic moulding materials (such as silicone or gelflex), in that to a certain extent it can be bent around undercutting and then retain its pattern shape. Any sort of clay can be used but bear in mind that red clays may colour the casting (which can be interesting). The consistency of the clay used is important, however. Too soft and the clay will break up and smear when released from the pattern; too hard and a good impression will not be achieved.

Generally plaster is used to create the casting from the clay mould because it can be produced quickly before the clay starts to dry out and no release agents are required. It is usually the case that a one-off mould is created, which is destroyed upon the removal of the first casting.

This mouldmaking method by no means has 100 per cent accuracy compared to the more high-tech moulding processes, and there are certain restrictions in terms of the amount of undercutting and detail that it will accommodate. However, these imperfections can produce unique and interesting castings. It is amazing just how much detail can be achieved with such a basic method of mouldmaking.

On a large scale, case-supported methods of press moulding should be employed. This is a method suitable for large-scale ‘site’ moulds, where the subject to be moulded cannot be moved and mouldmaking needs to be carried out ‘on site’. Slabs of clay are prepared and applied to the original to be moulded and backed up with a rigid plaster case. Moulds in multiple pieces can be created in this way.

PROJECT

Specification

A six-piece clay press mould with a six-piece plaster case from an oak tree. Circumference 3330mm, a section 200mm deep.

Time Required

Due to the nature of the process, press moulding on a large scale should be carried out in one operation, which in practice means completing the project on the same day. This needs to be considered when deciding on the scale of a project.

Materials

■ Grey clay (red clays tend to stain castings);

■ alpha casting plaster;

■ a water source (if not available you will need large tubs of water or jerry cans);

■ a heavy fibreglass mat;

■ talcum powder;

■ plastic sheeting;

■ string;

■ petroleum jelly; and

■ chalk.

Tools

■ A sturdy collapsible table;

■ a clay harp;

■ a plaster mixing bowl;

■ 2in brushes;

■ scissors;

■ a builder’s bucket; and

■ a permanent marker.

Method

SET-UP AND PREPARATION

As with any project, careful preparation before the start of the job is crucial. Knowing the material quantities and tools needed, undertaking repairs to the original and setting-up the working area will all help to complete the job accurately and efficiently. (Tip: make sure you have enough materials to complete the job before you start. Running out of material halfway through the job is at least a waste of time and, at worst, a loss of previously executed work.)

In the case of large-scale press mould production careful preparation is of particular importance because of the nature of the process. The project needs to be completed efficiently and quickly because clay moulds will dry out, shrink and distort if not cast into immediately after completion.

1. Put up a sturdy collapsible table near the project site to prepare materials. Ensure your water source and bucket for washing tools and equipment are accessible.

2. The area to be moulded should be defined with a chalk line, or in this case a string line.

3. Protect areas not to be moulded using plastic sheeting or heavy-duty dustbin liners.

4. Prepare the clay slabs for the first section. These should be of a size appropriate to the job. Their thickness will depend on the depth of detail to be captured, although they should be thick enough not to break through when pressed into detail. In this example the size of the slabs are 550mm × 200mm × 30mm, and they are applied two at a time by two people.

THE APPLICATION OF RELEASE AGENT

1. With a soft brush apply a liberal amount of talcum powder to the whole area to be moulded.

2. Brush off any excess talcum powder, leaving a fine coating of powder on the surface.

APPLY THE FIRST TWO SLABS

The first two slabs in this project are applied on opposite sides of the tree.

1. Press your pre-prepared slabs firmly to the surface of the tree. Use the heel of your hand to methodically press over the surface of the slab to push it into the detail.

2. If necessary apply the slab in sections butted up against one another. This is usually necessary with large pieces where it may be difficult to apply a section of the mould in one piece because of its size.

3. Create a neat clay wall around the outside edge of the slab. This is to retain the plaster when case making. The wall should be 15mm high along the top and bottom edge of the slab. It should be approximately 50mm high at either end of the slab to create a flanged wall to the case section that will join it to the next section of the mould.

4. Pinch the bottom outside edge of the wall onto the surface of the tree to keep it in place.

Site preparation and bench set-up.

Talcum powder is dusted on the area to be moulded.

The first clay slab is pressed onto the first section to be moulded.

A second slab is pressed up against the first to create the first mould section.

A wall is created around the slab to retain the plaster case.

A first layer of plaster is applied to create the case supporting the slab.

With the first mould piece complete, the end retaining clay wall is removed and release agent applied to the case flange.

The second mould slab is butted up against the exposed edge of the first.

Working on opposite sides of the site to create two mould pieces at the same time.

The last two mould pieces are created between the previously made pieces to complete the mould.

Holes are drilled through flanges to accommodate nuts and bolts to secure them upon reassembly.

The mould pieces are removed.

The six mould pieces, ready for reassembly.

The mould reassembled, ready for casting.

CASE MAKING

The clay slab will have picked up the detail and shape of the tree section but when it is removed it will distort so a rigid case is needed to enable it to hold its shape. The quickest way to do this is with casting plaster.

1. Prepare strips of heavy fibreglass mat. There should be enough to cover the slabbed section once.

2. Apply a layer of plaster 2–3mm thick to the surface of the clay slab. This will pick up detail of the clay slab. Apply plaster to the inside surfaces of the clay walls as well.

3. Allow it to set until it is not depositing onto your finger when touched lightly.

4. Apply a second coat to the first and allow this to set.

5. Dip strips of the fibreglass mat into a mix of plaster and lay them on top of the plaster you have already applied. Ensure a good lamination by pressing lightly over the surface to push out any air bubbles.

6. Apply a thick mix of plaster over the mat layer to complete the case.

7. Allow the casing to set hard.

THE NEXT TWO SECTIONS

With the first two sections of the mould complete, the next two sections can be created. These will be formed directly against the first two sections.

1. Carefully remove the clay walls at either end of the first mould section, revealing the flange wall of the case and the edge of clay mould.

2. Apply petroleum jelly to the exposed flange wall of the plaster case.

3. Prepare the clay slabs to create the next section.

4. Butt up the first strip tightly to the exposed clay edge of the first mould section and then continue the slabbing process in the same way as for the first section.

5. Create a 50mm clay wall at the free end of this slabbed section.

6. Create the clay retaining wall along the top and bottom edges of the second slabbed section. These walls should connect to the exposed edge of the adjoining first plaster case flange.

7. Create a plaster case as before, applying plaster directly against the exposed case flange of the first mould section.

8. Allow the casing to set hard.

THE LAST TWO SECTIONS

Next the last two sections of the mould will be created between the finished previously created mould pieces.

1. Apply petroleum jelly to the two exposed case flanges of the previously made mould pieces.

2. Create the final mould pieces using clay slabs as before, butting up the clay directly against the clay edge of the adjoining mould pieces.

3. Create the last two plaster case pieces as before, applying plaster directly against the flange wall of the previously made case pieces.

4. Allow the casing to set hard.

REMOVING THE MOULD

Once the mould is complete it can be removed from the tree section by section.

1. Before removing any sections, drill two 6.5mm holes through the two adjoining plaster case flanges between each section of the mould. These will accommodate M6 roofing bolts and wing nuts to hold the mould sections together when casting.

2. With a surform tool take back the top edges of the adjoining case flanges to expose the seam line between them.

3. Number or letter each section of the mould to aid reassembly. Also mark an arrow to indicate which way up each section should be when reassembled.

4. Carefully remove the mould sections one piece at a time from the surface of the tree. When doing this be careful not to smear the clay as it comes off. If while removing the sections the plaster case comes away from the clay slabs, carefully lay the empty case on a surface, remove the slabs separately and lay them directly back into the case. This can also be a good way to remove moulds that are deeply undercut, allowing the clay slabs to be bent past any undercutting.

5. Carefully lay each mould piece down on a surface as they come off.

6. When all the mould pieces have been removed reassemble them in order, a section at a time, tightly fastening the sections together with nuts and bolts.

7. Once the mould is reassembled it should be cast immediately to prevent shrinkage of the clay as it dries.

8. For the casting process, laminate the mould with alpha casting plaster and heavy fibreglass mat directly against the wet clay surface.

9. Demould the casting by removing the case pieces and peeling the clay slabs away from the casting.

10. Clean up the site and remove any rubbish and waste.

Alpha plaster is applied directly to the clay mould to create a laminated cast.

The demoulded cast.

Cleaning up the site.