Brick or sand hearths

In most smaller kilns the glass will be supported on shelves or batts above an insulating fire brick (IFB) floor, but in larger kilns the glass may be placed directly on the IFB itself; with only battwash or a texturing agent in between.

Less frequently the hearth structure will have an edging to allow the screeding and levelling of a sand bed into which imprints can be made.

Brick hearths

Kiln builders have taken different approaches to the design of brick hearths in glass kilns

• IFB can be laid loose on a slatted or expanded metal support so that they are free to move. This to prevent the bricks arching up in the centre during the firing should they expand more at the top than at the bottom. Any bedding powder or brick dust which gets between the bricks during a firing will fall out of the gaps and through the expanded metal as they cool and contract.
• Hollow concrete blocks (Besser style blocks) can be stood on end and packed tightly together as support for a sand bed. No IFB is used but FC sheet or fibreboard may be laid on the blocks to stop the sand falling into the gaps. Certainly provides a solid support, but the high heat storage could be wasteful of power and be slow to heat and cool down.

My preference is for a multi layer structure which minimises the temperature gradient across the bricks, minimises differential expansion. The design assumes that they will be securely restrained at the edges and the gaps between bricks will be filled with mortar or refractory powder.

Fig.16-01

Hearths in larger Riley Top-hat kilns use a multi layered construction as shown in Fig 1.

It has a single layer of IFB laid on the flat, supported on a 12mm layer of CF fibreboard. This rests on a 4.5mm thick layer of fibro cement which in turn rests on a heavy duty expanded metal floor. The support framework is made of square or rectangular hollow section steel tube (RHS) which is surrounded by a heavy angle iron frame.

The rationale behind this construction is as follows:

• The expanded metal is used to prevent the oil-canning or warping which could occur with a sheet metal floor. It is welded to angle or tube supports at close intervals to ensure it stays true. (Oil canning is where sheet metal heats unevenly and hotter areas expand more than the cooler metal at the edges, causing it to bulge.)
• The fibro cement is to prevent the fibreboard being abraded by the expanded metal; especially after the binder has lost its strength.
• The high temperature gradient across the fibreboard ensures a minimal gradient across the brick.
• The angle iron frame allows the IFB to be placed under compression so that expansion will be less that it would otherwise be if not restrained. This is something like the railway track across the Nullabor which remains straight and true because of the restraint of the sleepers and track bed; in spite of the wide temperature swings to which the metal rails are subject.

Mortar joints

Unlike conventional brickwork, with wide straight and even width mortar joints, the mortar joints in refractory brickwork should be kept as thin as possible. The ideal would be to have no mortar at all. See 73. Insulating Fire Brick for more.

16-02

With use, the top face of the bricks will gradually wear away. Most mortar used for jointing bricks is much tougher than the brick itself, and doesn’t wear away. It the joints were filled to the top they would eventually protrude above the bricks and be difficult to hide or remove. The lines would show through powder beds and be visible in slumped glass.

16-03

To avoid this, the mortar is best applied to the bottom of the joint, as in Fig 3.

Brick dust or bedding powder is allowed to fill the rest of the gap.

Brick layout

The dimensions of the hearth should be such that short or narrow pieces of brick are avoided as much as possible.

IFB available in Australia are 76mm (3 inches) thick and should be laid on the flat, in stretcher bond. Laying them on edge increases the thickness to 115mm (4 1/2inches) This would increase the temperature gradient across them, and also increase the thermal mass and heat storage. For more on this see 72. Elements of kiln design.

It is not advisable to make a hearth too thermally efficient. In one case a different firing pattern was required on Monday than for the rest of the week, when doing the same work. Heat stored in the hearth after the Monday firing was retained for the rest of the week and not lost until the weekend break.

Mortar

Mortars used for refractory work are different from those  used in domestic and commercial construction. That used in kilns for glass and pottery is called an ‘airset’ mortar. A readily available brand in Australia (in 2008) is Tufset Mortar, by Vesuvius Australia. It comes in 15kg pails, is a darkish colour with a fine texture and, when first opened will have a layer of liquid on its surface.  The liquid is sodium silicate diluted with water to about 10%. This must be thoroughly mixed back in, preferably using a mechanical stirrer and working the liquid back throughout the pail. Fig 16-04 An effective tool is a bent steel rod in a portable power drill, as shown in fig 4. Like most mortars, it thickens up fairly quickly if undisturbed (the initial set, which is reversible by stirring), so must be stirred every few minutes to keep it at its best working consistency.

Fig 16-05

To avoid having to frequently stir the whole drum, decant the quantity needed for the current work into margarine or similar containers and reseal the pail. The small containers can be sealed until required and are easily stirred using a paint scraper as a combined stirrer and applicator.

Most IFB are quite absorbent. They will speedily absorb the liquid from the mortar, making it too stiff and dry to conform and bond to the mating surface; a dry joint can result. Working fast and stirring the mortar frequently can usually overcome these problems.

If necessary, a VERY small amount of water can be added to the mix; less than a teaspoonful to a margarine container at a time if the mix is not to become sloppy, and mix it in thoroughly. 

Fig 16-06

For hearth construction, apply a small smear to the bottom of the long and short faces which will contact previous bricks and quickly push with a sliding motion into position so that the join will be as narrow as possible.

Fig 16-07

The entire surface should be sanded after laying, using coarse sand paper on a flat piece of timber or steel tube.Let the dust fill the joints, with some alumina or kaolin added to fill the finer gaps.

Hearth frame

The hearth frame must be of such robustness and rigidity that it will withstand the sideways pressure of expanding brickwork, as well as any load to which it may be subjected. In wide hearths this can include the need for the operator to sprawl on, or even walk on, the hearth. 

Hearths for top-hat kilns

Fig 16-08

To prevent stressing the edge of glass when Rapid Cooling, a depressed edge should be provided on all sides of the hearth where cold air can enter. This will ensure that the entering air will be deflected upwards and not impinge onto the edge of the glass. Fig-8

Fig 16-09

This can be easily achieved by using a split brick for the edge and rebating the outer row of hearth bricks to fit over them.

The gap at ‘A’ can be filled with mortar. When this sets it will act to provide the restraint against expansion mentioned previously.

Even though the edge brick is thinner than the hearth, and thus provides less insulation, little heat will be lost through the gap between hearth edge and hood as it is quite narrow; ideally no more than 20mm.

Sand beds

Commercial operators often use a sand bed into which patterns or designs can be imprinted as the working face of their hearth. As most designs aren’t excessively bold, a sand depth of about 10mm is usually sufficient.To achieve a level surface, some form of ‘screeding’ edge should be provided.

Fig 16-10

One cost effective solution to this, incorporated in the composite  hearth design above, is to use strips of 10-12mm thick silimanite kiln shelf inserted into a slot around the hearth proper as in fig 10

There is a limit to the size of shelf available, so the length of strips will be short and a number must be placed end-to-end along a side. The strips will expand and contract with each firing and sand will work into the gaps created; gradually increasing the overall length of the edge and possibly exerting damaging pressure on bricks at the end. Periodic removal of strips and cleaning out of accumulated sand will overcome this.

Maintaining a hearth

Be gentle when removing a powder bed between firings. Scrape up the bulk of the powder with a shovel, but avoid heavy brushing as this will accelerate the wearing away of the brick surface.

It is almost inevitable that a hearth will crack at some time. Other than for appearance, cracks are little more than ‘additional joins’. Let them fill with bedding powder.

Only in exceptional circumstances should mortar be used to fill them.

In the event that a piece of brick break loose, mortar it back in by putting mortar on the bottom and lower edges, press in tight and, when dry, sand back to level and allow surface gaps to fill with powder.

In an emergency one can make a mortar from kaolin or battwash powder mixed to a thin paste with colloidal silica.

Like everything else, the hearth will gradually wear out and need replacing. Don’t expect it to last forever. Inspect it regularly, repair damage promptly, but treat it as a ‘tool’ to help you produce the best results.