Kiln Furniture

                    

    Kiln furniture is the furniture used by the ceramics industry to support wares and create more space during firing. It refers to a wide range of refractory products used to support and protect the products during high-temperature processes. It is actively used in the ceramics industry and beyond, in applications such as powder metallurgy, glass, composites, and even bakery and pizza ovens. It is traditionally used in the fire of tableware, sanitaryware, and heavy-clay items.

    The kiln furniture plays an important role in the furnace's heat management. The kiln furniture has a huge impact on the charge's interaction with the furnace atmosphere. In order to manage gas exchange, chambers, in which the heating material is embedded, are widely used especially in addition to different carriers and plates.

Kiln Furniture requirements

The main requirements of kiln furniture are as follows:

•  High Pyrometric Cone Equivalent (PCE)
•  High Hot Modulus Of Rupture (HMOR)
•  High Thermal Shock Resistance (TSR)
•  High service life
• Dusting should be low

Kiln furniture construction

    Kiln furniture is either made of Silicon Carbide or Alumina. The types of Silicon Carbide and Alumina are as follows:

For Silicon Carbide

• Oxide bonded Silicon Carbide
•  Nitride bonded Silicon Carbide
•  Recrystallized Silicon Carbide

Recrystallized Silicon Carbide has better quality when compared to other two types of Silicon Carbide.

For Alumina

•  Cordierite
• Mullite
•  Combination of Mullite and Alumina
•  Low purity Alumina High purity Alumina

    The percentage of alumina content in the above mentioned different types of Alumina are 45% Al₂O₃ (Cordierite), 60-65% Al₂O₃ (Mullite)_, 88-91% Al₂O₃ (Combination of Mullite and Alumina)_, 95% Al₂O₃ (Low purity Alumina) and 99% of Al₂O₃ (High purity Alumina).

    As the quality of Alumina increases, automatically the cost, operating temperature increases and Strength increases. The main disadvantage of using Alumina is lower thermal shock resistance as the alumina content increases in the types of alumina, but in case of decreasing alumina content have higher thermal shock resistance.

Comparison of Silicon Carbide and Alumina

S.No	Silicon Carbide	Alumina (High purity)
1	Lower cost	Higher cost
2	Operating temperature is 1650oC (Beyond this temperature, it oxidizes)	Can be operated at higher temperature
3	Degrades in Oxygen atmosphere	Do not degrade, because it is already in oxide state
4	Reacts with Bismuth containing material	Unreactive
5	Higher strength	Relatively lower strength
6	Higher Thermal Shock Resistance	Lower Thermal Shock Resistance

    Although some types of ware can support and protect themselves when stacked in a kiln, e.g. bricks, Multiple kinds of support and protection are utilized for a variety of reasons. Now, let's see the variety of protecting and supporting materials in a detailed manner

Kiln furniture as a protecting material

Kiln furniture, used to protect the product or safeguard ware from open flame, smoke, gases, and kiln debris.

Example: Saggars

Saggars are the boxes in which the product is kept and fired. They are available in different shapes such as flat, oval, round, and rectangular shapes.  The main purpose of saggars is to protect the articles from direct flames, ashes, sulphurous gases etc.,

Kiln furniture as a supporting material

Kiln furniture, used to support and type of product to be fired. It is usually removed from the kiln or furnace at the end of the process.

Products in supporting material

1.     Table ware 

2.     Brick 

3.     Kiln furniture for sanitary ware

4.     Electrical porcelain 

5.     Electronic ceramics 

Let’s discuss briefly about the variety of products in supporting material

1. Table ware

    a)   Commercial china

    b) Bone china

a)     Commercial china

Commercial china undergoes two process of glazing which are Bisque firing and glost firing.

Bisque firing: Apply before glazing. In bisque firing, setters is like similar to wave, stacked one above the other and placed in bird cage system (support). Settler is made up of Silicon Carbide (SiC).

Glost firing: For glost firing, the shape of the base is V shape or moon shaped, three upright port and many small triangular pins are available. These pins are fixed in holes. There is a contact in pin region, so there may be a chance to stick. Hence they are made up of coerdierite.

Advantage of triangular pin structure design:

• Low refractory to load ratio

It means less amount of refractory is used for load. Due to this, an advantage includes cost and mainly the energy is used for firing. If the amount of refractory is higher, energy is higher and energy is lower for minimum amount of refractory.

 Disadvantage of triangular pin structure design:

• Formation of puck marks (pin support may have puck marks)

                    

      

        To overcome this puck marks, triangular base with three legs are used. The advantage of triangular base with three legs is to avoid the formation of pucks. Coming to its disadvantage, refractory to load ratio is high.

b)    Body China

Body China undergoes two process of glazing which are Bisque firing and glost firing.

Bisque firing: In bisque firing of body china, setters were used. Setters were made up of coerdierite. The problem involved in this type of firing is high refractory to load ratio.

Glost firing: Glost firing of bone china is made of hooded setters and made up of coerdierite.

    Bone ash is a good fluxing agent which is having the tendency to slump. So, they are made of hooded setters to avoid sulphurous gases. Due to this kind of setters, at firing atmospheric gases does not affect the product and hence achieved good glossiness.

2. Brick

    Kiln furniture is used for firing bricks and bricks are made up of coerdierite. Post is the base, above the post batt is placed, above the batt, slab is placed and finally above the slab, brick is placed.

3. Kiln furniture for sanitary ware

    For sanitary ware, kiln furniture is made up of recrystallised SiC since, it has good thermal conductivity, good strength and less weight.

4. Electrical Porcelain

    An example for electrical porcelain is Metal oxide varistor (Bismuth). It is fired either with coerdierite, mullite or alumina. It is hanging between the post. Saggar based heating is required for this product. In ancient days, porcelain is made with oxide bonded SiC. But, currently made with recrystallized SiC, due to its better quality.

6.     Electronic ceramics

    For electronic ceramics, hard ferrite and soft ferrite is used.

    Hard ferrite: Hard ferrites are fired in roller hearth kiln. For this kind of ferrite, lin car and kiln furniture are not required. In roller hearth kiln, plate are made up of coerdierite.

  Soft ferrite: Soft ferrite requires controlled atmosphere and increasing firing temperature. In case of soft ferrite, kiln furniture is used for better quality than coerdierite. In earlier days, Alumina based-SiC core or Duplex type is used. Alumina (Pure) is expensive. So, SiC core or duplex type is attached to alumina face to provide good strength, unreactive, better performance and inexpensive. The examples of soft ferrite are: Capacitor and individual setters.

Capacitor

Setter is used for firing the capacitor. Zirconia setters are used in ancient period.

Advantage of zirconia setter:

• High quality product with low rejection rate

Disadvantage of zirconia setter:

• Expensive
• Production range is low

    To overcome this problem, now a days capacitors are made by saggars with mullite or alumina with Zircon parting agents.

Advantage of saggars with mullite or alumina with Zircon parting agents:

•         Production rate is higher
•          Lower firing cost

Disadvantage:

•            Higher rejection rate

    Individual setters are also available. They are made by the body of mullite thermally coated with Zirconia for better performance setters to avoid deformation.

Advantage of Individual setters:

•         Better production with lesser rejection

 

    Kiln furniture, which is designed to keep the components from deforming or bonding to one another. The charge must also be heated and cooled as homogeneously and precisely as possible, whatever of its position in the furnace, to achieve good product quality and a minimal rate of rejection. In this article, we discussed regarding kiln furniture especially used in the field of refractory. I hope that you will gain some idea about this topic in ceramics.