Making a fibreboard mould

Here is the procedure for making the mould for the platter previously shown.

The drop is 25mm, so two layers of 12mm thick fibreboard are being cemented together to give the height.  The piece of glass is 400x 250 x 6mm, so the pieces of board should be at least 10mm larger in each direction to properly support the edges.

Fig 77-01

Two pieces of fibreboard are cut to size. A paper template of the mould is made and the outline of the cutouts is transferred to the two board pieces.

	77-02 Pre wetting the board
	77-03 Applying mortar
	77-04 Applying top layer of board

The two pieces of board must be stuck together. Figs 2, 3, and 4 show the procedure. In this case a refractory mortar used for jointing brickwork is on hand, but for most people one of the made-up brews mentioned in 61. Mortars and adhesives will do the job equally well.

The dry board will rapidly suck the moisture out of the mortar, so it should be well wetted where the mortar will be applied. Fig 2. This will ensure that the mortar will still be moist and flowable when the top board is pressed into position.  

The mortar is well mixed in a small container and applied within the lines as speedily as possible to ensure that the first applied doesn’t dry out too much. Fig 3.

The second board is applied and pressed into place to squeeze the mortar layer out to as uniform a thickness as possible. Fig 4.

To ensure the mould will be flat it is good practice to place the boards on a flat surface and apply a load until the mortar has reached its initial set. A good idea is to have available a couple of equal size pieces of 12mm or 18mm thick chipboard which can be used with bricks or other weights to make an improvised press. Sand the edges and coat them with a waterproof paint to ensure that they stay clean and smooth. Water soaks into chipboard and makes it swell.

	77-05 Pilot holes
	77-06 Using blade
	77-07 Inclining the cut

When the mortar has set the cutouts can be made.

First, drill holes in two opposite corners of each of the pieces to be removed. Fig 5. The holes should be in by a few millimetres’s from the marked lines and be of a size that will accept a hacksaw blade.

Next, take a hacksaw blade and, starting in one of the holes cut out toward the line and then along the line to the corner. Then reverse the blade in the slot and cut to the other corner. Repeat for all other holes.

To avoid the risk of the glass being trapped in the mould, hold the hacksaw blade on a slight angle to the perpendicular so that the sides of the holes will taper inwards toward the bottom. Not much of an angle. Fig 7 gives some idea.

Hacksaw blades come in three pitches or teeth per inch; 32 fine, 24 medium and 18 coarse. I prefer to use medium or coarse blades on fibreboard, and to hold the blade so that it cuts on the up stroke and away from me so that the line is easier to follow.  Don’t rush, be gentle. Make sure the board is well supported. It can break fairly easily

When all the cuts are made, the faces can be cleaned up and the top edges slightly rounded. Faces of the cuts can be straightened and smoothed using a file, a rasp or a piece of abrasive paper of about 80 -120 grit wrapped around a sanding block.

Give a gentle rounding to the top edges of the cutouts. The actual shape of the rounding is of less importance than that they are all the same shape, as variation can give different appearance to the edges of the depressions on the slumped glass.

Any nicks, chips, breaks or cracks in the areas to be contacted by the glass can be built up using a mixture of kaolin or battwash powder with colloidal silica as a binder. See 61. Mortars and adhesives for more.  Allow the repairs to dry, finish sanding.

When all is cleaned up, apply one or two coats of colloidal silica by brush or spray, drying between coats. Applying by brush is most convenient as there is so much cleaning up of equipment to do after spraying. Be sure to wash the brush out thoroughly after use, otherwise it could go rock hard.

Last step. Battwash.