Uniformity of shrinkage over the kiln cross-section Attainment of uniform firing is especially difficult

where kilns have wide inlets and are used to produce large tiles. The extent of shrinkage and, more

importantly, the body vitrification range, are crucial in highlighting the effects of even small

temperature differences across the firing channel. Temperature differences can be quantified and

pinpointed by measuring pieces pressed in the same die cavity and fired in different load positions

(all tiles must be oriented the same way with respect to the press outlet). After marking the tiles to

indicate their position (fig. 26) all the right and then all the left side pieces are compared.

An example is evaluated as follows: + 0.5 mm on left side of kiln with respect to middle tile + 0.2 mm

on right side of kiln with respect to middle tile. An indistinct comparison between right and left sides

includes pressing errors (0.4 mm on right-hand side of piece). When checking the tile dimensions

comparisons should be made between tiles all across the kiln section and care should be taken to

note any effect resulting from pressing errors which overlap with firing variations.

a) The most frequent occurrence is larger tiles in the cooler areas nearer to the outside edges of the

kiln cross-section. Several causes combine to create the problem (fig. 27):

– Flame temperature at the nozzle is lower than further away where combustion is complete. – Re-

circulation of colder ambient air on account of the Venturi effect generated by flame speed. – Fumes

move more slowly near the walls than in the middle of the channel because of friction (flow towards

chimney). – Stronger irradiation of centre of the load by perpendicular walls. – Outward dispersion of

roller heat by conduction. – Aspiration of cold air via improperly plugged roller seats in the firing

zone. – Low-speed cold air blow movement, especially above rollers. The problem can be corrected

on several fronts (fig. 28).Kiln pressure (see relevant paragraph) must be positive in the firing zone

(at roller height) so as to prevent cold air infiltration; however, pressure should not be too high. –

Insertion of one or more deflectors at the beginning and inside the firing zone diverts more fumes

towards the tiles near the walls. – Using a suitable number of burners with semi-radial dispersion

above and below the rollers is a tried and tested system that compensates for colder flame areas

near burner nozzles and any intake of colder ambient air. To fine-tune, air can be dosed to the

burners to modify flame temperature and length. – Flame deflector screens can also be positioned in

front of firing zone burners beneath the rollers (fig. 29). b) Irregular shrinkage across the cross-

section (known as “candy paper”). Characterised by shorter tile sides 60-70 cm from the walls and

longer ones adjacent to them (fig. 30). Here, despite the similarities with case a), erroneous flame

geometry (especially on burners above the rollers) is usually the culprit. Considerable modifications

to flame speed and geometry are usually required. An easy and often effective solution is to shut

down a burner in those sets that markedly influence size; alternatively, the flow of burner air can be

modified. c) Other, differently configured, irregularities can also occur; depending on the situation,

their solution may require use of the above solutions or even their direct opposites. Glaze brilliance

and shade In terms of shine, satin effects, opacity, spread and shade the final outcome of glazing

essentially depends on the high temperature firing stage. Higher temperatures or extended firing

time results in more compact vitrification of the glaze and thus improves shine. Longer times at

relatively lower temperatures give a satin look and better (flatter) spread. Much, of course, depends

on the nature of the glaze, the limitations of which are inevitably highlighted by pin holes and micro-

bubbles if temperatures are too high. The appearance of satiny effects (clouding of the glaze at

certain points of the load) indicates a combustion defect on one or more burners in the high

temperature zone; a check on (and possible increase in) the flow of combustion air is generally

required. Shade changes across the load are the result of temperature gradients.