In general, insulated firebrick serves only two purposes in the glass kiln. They prevent the heat from escaping from the kiln (insulate), and they support or retain elements. If the brick serves both of these purposes, then the brick remains functional and does not need to be replaced but may require some repair.
However, glass kilns present two additional important nuances with brick: dusting and overfired glass on the bricks. Brick dust is a bane of the glass artist. Brick dust or small chunks can make a beautiful piece of glass art an exercise in frustration. Glass melting into firebrick is normally not a terminal event for a kiln, and it can be repaired.
Before proceeding to specific examples, some understanding of the brick material itself and its characteristics is helpful. Insulated firebrick is a combination of special clays mixed with other materials to give the porosity seen in the bricks. This porosity makes them light weight and have good insulating properties. The mixture has a cake batter appearance and is poured into molds to dry. The bricks are removed from the molds and fired to 2300-2400F in a tunnel furnace. They are manufactured in thicknesses of 2.5” and 3” and capable of withstanding temperatures of 2350F for extended time periods. This means most art glass application temperatures are far below any maximum rating of the firebrick material.
Firebricks can have voids or cracks or impurities like metal oxides in the finished product. Some voids or cracks are not visible and can result in certain portions of the brick being structurally weak. As bricks heat up, they will expand at their coefficient of expansion. The bricks also produce brick particles or dusting.
The dusting issue is normally mitigated by coating the brick surface of the lid with a mixture of brick cement, water, brick dust and a gumming agent. This brick coating is combined to a consistency similar to that of cream. It should go on the brick like a wood stain in that it soaks into the brick and hardens the surface. The grain of the brick should remain somewhat visible. If the coating is either mixed too thick or applied too thick like enamel paint, it may chip off the brick. This coating can be reapplied if it abrades away. The brick cement, the main ingredient, has a different COE than the brick itself.
The most common brick defect on glass kiln lids is chipped brick at the outer corners or edges.
As the bricks increase in temperature, they expand. The only place for the bricks to expand is toward the edges of the lid. The corners of the lid are the furthest point from the center of the lid, and the brick corners push against the metal band on the lid. If the brick has any structural weakness from voids or cracks, that is where the brick will chip or break. These chips should not concern the glass artists. There is negligible loss of insulating capacity with the loss of so little brick material, and this portion of the brick does not retain any elements. It is suggested that the chip be recoated if the chip is on the bottom of the kiln lid where there is potential dusting onto the artwork. Do not try to fill the void with kiln cement. The cement will eventually break out and take more brick with it.
The most common problem with the kiln floor and walls is surface cracking.
These cracks should be of no concern as they are not retaining the elements nor impacting the insulating capability of the firebrick. The cracks are generally on the surface and do not go through the brick. Moreover, when the kiln heats up, the brick expands to close the cracks. These should be left alone. Do not try to fill the cracks with cement as the cracks will get larger and the cement will break out. If a crack is large enough to warrant replacement, there will be damage to the outside metal of the kiln where the heat has been able to reach the metal. In this instance, the burn mark will be very apparent on the outside of the kiln metal. This is extremely rare.
The next brick issue is glass getting onto the floor of the kiln.
This type of damage is not reason for replacing the brick in the kiln. Normally, the glass has only gone into the surface of the brick less than one inch. Most kiln floors are at least 2.5” thick and many are 3” thick firebrick. The glass can be removed with a putty knife or a pocket knife and some patience.
This photo shows the floor of a large glass kiln that had damage from molten glass, and the glass was removed. The kiln remains operational. Some areas are not flat enough for placing kiln posts, but the posts can be placed around the damage to still support shelves. If the floor is damaged to such an extent that posts cannot be placed on it, shelves can be placed on the damaged floor to make a new floor that is sufficiently flat. Do remove all the molten glass from the kiln walls and floor. The glass can cause more damage later if not removed.
A brick issue that does require immediate attention and probably replacement is when the brick supporting the lid element breaks and the element is falling from the lid brick.
This condition may be repaired in the short run by using element pins or staples to hold the element in place, but the lid may need to be replaced for it to operate well over the long run.
Hopefully, this article explained how to address many of the common brick issues encountered by glass artists and how to decide which brick problems warrant attention. Just because the kiln is cosmetically challenged with worn paint and every brick is cracked, does not mean the kiln will not fire like a dream.
This article appears in the July/August, 2009 issue of Glass Art magazine.