To have an efficient clay phase preparation, the blunging system (power installed , propeller type, sieving capacity) and the clay type have to be compatible.
The potential issue is mainly a difficult sieving due to UN-dispersed clay particles, leading to inconsistent formula, wastes of good material, longer blunging and sieving time. A high residue level in the clay could lead to similar issues.
Difficulties to adjust the density
Basically, sanitary ware casting requires high solid percentage (density around 1800-1820 g/L for vitreous china, and 1960-1980 g/L for fine fire clay) together with good fluidity. The slip has to present low viscosity with adjusted thixotropy, in order to cast difficult and complex pieces with good yield.
In some cases, the use of poor quality or un-adequated plastic raw materials (clay with high illite or smectite / montmorillonite content) could avoid the achievement of the proper rheological parameters.
Too low density or too high viscosity
In order to increase the density, we recommend to use products with higher fluidity, that will enable to reach higher solid content of the casting slip. The same recommendation could be done if an increase of deflocculant do not help reaching the proper viscosity, or if the slip contains too much deflocculant, causing casting cords, mould corrosion.
Too high thixotropy
A too high thixotropy is generating many issues during the casting process of sanitary ware pieces: soft and flabby pieces causing bad drainage, deformation and cracks.
When the thixotropy could not be adjusted with the defloculants, you have to look to soluble species (salts), like sulfates (SO₄²-, Cl-, NO₃-…), coming either from the water (and especially if it’s well water), or from the raw materials themselves. The return slip is potentially contaminated with plaster from the moulds, bringing sulfates, and giving thixotropy.
Ageing is an important step to stabilize the rheology of the slip before casting. Nevertheless, some important issues could appear during the maturation of the slip :Slip sedimentation:
Slip sedimentation usually comes from too low density. Apart from accidents like excess of water, and if an increase of density is not possible (slip already at optimal defloculation), we recommend to replace a low fluidity clay or kaolin by a material with better properties:
The low thickness formation, or low casting rate, founds its origins mainly in a bad packing of the particles in during the consolidation. The main actions to increase the casting rate are:
A bad drainage of the mould after casting is leading to surface defects :
drips on visible surfaces, that with impact the glazing, and remain visible after firing;
slip hang-up that will potentially give cracks during the drying.
A better adjustment of the rheology, and especially a reduction of the thixotropy is mandatory to increase the drainage.
The demoulding of the piece is a critical stage, because the operator will have to handle a soft piece. If the slip properties are not properly adjusted, and the technological times not observed, some issues could appear in here, and sometimes only revealed after drying or firing :
A too high thixotropy is usually the origin of too soft pieces, together with a too low density and over- deflocculation.
A better adjustment of the casting parameters is necessary.
The cracks during the demoulding stage have 2 main origins : lack of plasticity of the cast piece and too high shrinkage in the mould.
Lack of plasticity: the plasticity is too low to tolerate the constraints given to the piece during the handling. We usually recommend to increase the plasticity of the clay, increase the clay percentage in the formulation, or use a more plastic kaolin :
Too high shrinkage in the mould: the shrinkage of the cast piece is starting in the mould, but the lower part of the mould is any shifting. Thus, the piece is contracting itself on the mould, and some cracks can appear. The easiest way to solve this issue is to increase the slip density. Less water in the slip means less shrinkage.
These defects are directly linked to the interaction with both surface of the slip, air and plaster. During the mould filling, the surface of the slip has higher concentration of fine and/or colloidal particles, and also electrolytes.
When two fronts of casting slip are joining, this heterogeneous surface with a creamy aspect could form a slip meet, an improper joined plan, leading to a potential defect.
The rheological set up of the casting slip remains the main driver to control and avoid these defects, usually generated by a too fluid or low thixotropy slip.
Air bubbles trapped close to the surface of the cast piece are coming from:
Too fast stirring of the slip before casting
Too high viscosity avoiding the migration of the bubbles before casting
Numerous defects in the sorting after firing are generated by the previous steps of the process: casting, drying, glazing.
After the first firing, the shape of the crack could tell about the origin: if the crack is sharp, the origin is the cooling. If the crack is smooth, then the origin is the pre-heating or heating.
For an issue in heating or pre-heating, the potential issues are a too high moisture due to incomplete drying of the piece. Otherwise, the pre-heating curve has to be adjusted.
For an issue in the cooling, if the firing curve could not be adjusted, we can consider that the best way to solve the issue is to decrease the thermal expansion coefficient. The total quartz amount could be decreased either by a reduction of the quartz itself, or by using higher alumina content clays:
Creeping (Deformation or Pyroplasticity)
Either in vitreous-china or in fine-fireclay, creeping is a key parameter to keep under control.
Indeed, a variation in the creeping could have an impact on the geometry of the product itself.
To reduce the creeping of vitreous china:
Reduce total amount of feldspar, and prefer K feldspar to Na feldspar, or reduce firing temperature, but always ensure the water absorption is still correct
Use more refractory plastic materials (clay and kaolin)
Increase green density of the cast piece – better rheology
An initial interruption in the continuity of the thickness of the glaze.
A successive influence of the surface tension (increased) during firing.
There might be many causes for this problem:
Excessive fineness of the particles (too long milling)
Too thick layer of engobe
Too fast heating of pieces when still wet
Irregularities (coarse particles or granulates) in the surface of the body, dust, oil, grease, casting marks (different permeability)
Re-absorption of moisture after drying
Soluble salts on the body
Insufficient or degraded binder
Powdery first layer of glaze, excessive thickness
Pinholes origin is a gaz migration from the body, either trapped by the glaze, or released too late, thus the glaze has not time enough to close. Could be an excessive degasification of soluble salts like sulfates (SO₄²- ), carbonates (CO₃²- ), organics, under-firing of the glaze, under-firing of the body.
The first corrective action is an adjustment of the firing curve and cycle, in accordance of the root cause.
The remedies could come from the glaze, with an adjustment of the fusibility (Raw materials for glaze), or from the body with a reduction or substitution of material containing salts or carbon, especially clays.
Crazing and shivering or peeling of FFC
Crazing consists in the appearance of network of cracks in the glaze.
Usually, crazing is due to improper glaze/body thermal expansion coefficient matching: the thermal expansion of the glaze is too close or higher than the body. An adjustment of the dilatation of either the body or the glaze is required.
Decrease the thermal expansion of the glaze: increase quartz and/or decrease feldspar.
Increase the thermal expansion of the body: this is the simplest action : addition of quartz (in 1% increments), to the detriment of another material (usually kaolin) or using a higher dilatation Chamotte.
If the thermal expansion is properly adjusted, but the autoclave test still giving crazing, it is possible that the moisture expansion is too high. Then crazing can appear after several month/years of use.
The main cause could be an excess of feldspar or alkali in the body, too low firing temperature or poor quality chamotte.
Shivering or peeling is the opposite phenomenon. When the thermal expansion of the glaze is too low compared to the body’s one, this could lead to detachment of parts of the glaze from the body.
Black spots / Stains
If blue stains are coming from the blue pigment used for the brightening of the glaze, black spots are usually coming from unclean glaze raw materials (contamination with iron).
Green spots are generated by a poor quality chamotte contaminated with pyrites or copper. Also the process has to be checked: brass connectors, pipes…
Brown stains are coming from manganese, usually from the water, but possible contamination of the raw materials could not be excluded.
Popping / chipping
Popping or chipping is a defect where a small chip of the piece (body + glaze) is blasted during firing and just sticks a few mm away.
This phenomena is usually linked to a contamination of carbonate or sulfate in the slip: gypsum from the mould, contamination of the water, calcium carbonate in a raw material and generally poor calcination of the chamotte.