SPECIAL VITREOUS ENAMELS “READY & EASY” TO APPLICATE, TO GAIN NEW PERFORMANCES AND NEW MARKET SHARES
Attilio Monzio Compagnoni - Wendel Email Italia S.r.l., Italy
Romano Ferrari (Consultant) - Wendel Email Italia S.r.l., Italy
Introduction to "preworked enamels"
"Pre-worked" enamels enable frit and enamel producers to offer enamelling shop customers a greater share of the service market at a set cost, together with their frits and mill addition articles, and relieve them from a lot of operations.
All this translates into economic, logistic and ecological advantages such as:
reduced operator hours for colour and effect selection and setup,
less machinery and plant, such as mills, vibrating sieves, magnetic separators with reduced energy consumption and maintenance costs,
minimised storage of raw materials such as suspending, opacifying, colouring mill additions and the like as well as plant and machinery parts,
less enamel rejects, mill washing water and their physical-chemical purifying processes that generate waste water, solid wastes and connected waste disposal issues,
less operating errors during weighing or mill operations that generate rejects or expensive corrective action.
Figure 1 shows the flow chart complete with operation steps and controls for the types of pre-worked products. With so-called "ready to mill products" (Cf. Flow Chart 1 of Figure 1) the enamel producer supplies the enamelling shop customer with just the weighted batch inclusive of frit and mill additions, ready for introduction into the mill, together with the required amount of water.
In this case, service offered is only limited to weighing the frit(s) and variousmill adding. Despite this reduced service, this type of pre-worked products has been well accepted in Italy, even more than the dry milled ready-to-use enamels.
We believe the reason is the progressive lack of specialised personnel and experienced enamelling department managers: the entire responsibility for enamel composition is in fact taken by the frit producer at a slightly higher but constant and well known cost, extra-cost being covered by cost reduction pursuant on personnel shortage.
"Ready to use" products (Cf. Flow Chart 2 of Figure 1) are dry pre-milled to the required fineness by frit and enamel producers who market the powders obtained to enamelling factories.
These mix and disperse powders in water with special screw impellers; high water dispersion pigments can be added during the dispersion of uncoloured powders in water.
The advantages of dry pre-milling are quite a few: time and operating hour savings for weighing, loading, running (3-4 h) and unloading operations, less enamel loss during mill washing, and less solid and liquid waste to be purified.
Nevertheless, dry pre-milling was not all that successful in Italy.
"Ready & Easy" products (Cf. Flow Chart 3 of Figure 1) are wet milled and controlled by the frit and enamel producer that also completes all preapplication tests and firing in the same conditions as the furnace of each Customer as well as any correction required.
When all tests comply with pre-arranged standards, the liquid enamel is forwarded to the customer who only has to assess consistency, add a small amount of water and mix.
Off all "pre worked "products, "Ready & Easy" enamels contain the highest percentage of added service, which obviously also entails higher cost. Sinceit covers special niche products, the cost/benefit ratio is no doubt favourable to the User.
Suffice it to consider the mere cost for developing a three-colour granite or a Metalised enamel.
The table of Figure 2 shows the summary of advantages offered by each kind of the three types of products, to better illustrate all the advantages featured by all pre working types.
Types of "Ready & Easy" main products
The main products of the "Ready & Easy" line considered in this paper are:
Polychrome "granite" enamels
Reduced bubble enamels
Polychrome granite enamels
The field of home cooking and heating appliances features maximum
resistance to the efforts of replacing vitreous enamel with various new
protective coatings, where the vitreous enamel won unquestionable
leadership due to scratch, heat and detergent resistance.
Attractive varieties of colours and surface finishings could also always be
obtained with enamelling.
A few years ago, enamelled washbasins were replaced by their stainless
steel counterparts or basins pressed from synthetic polymer sheets containing
polychrome inorganic granules.
The coloured granules give the surface an attractive look, with the
appearance of natural granite or a special ceramic product (such as glazed
granite or granite porcelain gres floor tiles).
Pressed synthetic polymer washbasins are launched on the market with an
increasingly elaborate mix, to follow modern colour hues or imitate natural
stone. Different colour and size granules are used for this purpose.
In this situation, there comes the demand to match enamelled cooking plates as well as the oven outside with these new washbasins.
Somebody might now believe porcelain enamel lost its leadership and surrendered to synthetic polymers, but this was not to be.
This is proven by the fact that porcelain enamels of the "Ready & Easy" line can easily achieve an aesthetic appearance equal to those of synthetic polymer items maintaining all its peerless technical and functional properties at the same time.
Any comparison would no doubt favour porcelain enamel.
Enamellers now have to decide to either accept the challenge and:
Attempt difficult enamel set-up (with little time available)
Perform a special application with 3-4 layers of granules on a base enamel and one or more firing(s)
Accept a remarkable percentage of re-worked items and a certain percentage of rejects.
Give up the order, which only actually covers a few thousand pieces!
Producers of high quality cooking appliances cannot however refuse such an opportunity, whence the request from enamelling shops to F.&E. producers to buy a single type of enamel containing all different colour and size granules, to be used with the process 1 coat/1 fire easy to spray and at acceptable cost.
This demand was accepted and satisfied by us.
The solution proposed by our technicians in Dalmine lies in:
A good over-firing resistant base-enamel containing calibrated coloured granules encapsulated into the slip.
Polychrome calibrated granules obtained from frits coloured during smelting, quenched through rotating rolls cooled inside with running water and subsequently broken between rolling crushers and finally sieved with a series of calibrated sieves.
This enamel is a typical "Ready & Easy" product guaranteed by the F.&.E. producer.
a) The base enamel should resist thermal shocks without spalling or crazing,
b) The granules prepared as above should be calibrated to the size requested by the Customer,
c) Granite enamels should be prepared in the required amounts in a short time,
d) Polychrome granite enamels should be tested in a Pilot laboratory continuous kiln capable of eproducing the firing curve of the customer's production kiln. It is the only way that the specimen prepared, fired and tested in the laboratory will correspond to the corresponding specimen obtained with the customer's kiln,
e) Have technicians with great enamelling Laboratory or in Shop experience and skill
f) Have technicians capable of training customer operators on proper application of this enamel, if required.
a) Base enamel
We do not intend giving information on how to prepare base enamels, as this meeting is attended by technicians with long experience and skill in preparing them; however, since the main problem is incorporating granules of 3-4 sizes into one polychrome enamel, we prefer to illustrate some features andpeculiarities.
We need first class products only supplied by leading producers. Moreover all frits, pigments and mill additions should guarantee stable enamel colour under possible temperature variations.
As firing conditions, particularly by jobber enamelling shops, can be subject to unwanted variations, the small batches to be granite enamelled and the variable reproducibility of production lots only enable the best granite enamels to feature colour stability.
It would for instance be wrong to formulate a "hard" base enamel to have good colour stability, as hairlines, caused by drawing-strains and appearing as Lüders lines can be generated. This defect can be particularly unwanted as it only appears after second firing, which can correspond to decoration or finishing firing. Items in fact do not allow for correction or
re-enamelling and should be rejected.
Pigment selection is critical too, and only long experience can dictate the best choice. Sometimes even mill adding selection of quartz, clay or electrolytes can play an important role in eliminating or enhancing some application or firing defects.
The most used colour hues used for base enamels range from ivory to beige, or from ancient pink to dark grey, or black with various colour shades. Conversely, to emphasize the base enamel, granules strong beige, ancient pink, ochre, leather, red brown, light brown, blue green, dark brown and black.
b) Coloured graniti - General information
The most common granite, such as the black and white for instance, were for years obtained from existing black and white frits.
These can easily used to obtain the simplest "micro granites" with good results: we thus get white micro granites on a dark background and viceversa.
An active master enameller, of the few who still remain, often tells of how a bag of white frit was erroneously put into a mill containing a ground coat batch; the resulting grey enamel was successfully used as a base enamel for oven insides. This grey enamel characterized a particular kind of range for years.
Standard frits are seldom used today for obtaining coloured granules, as many problems arise when producing granules of the required colour and size without a special frit (figure 4a and 4b).
b1) Size of the graniti in the base enamel
Grain size is obtained by acting in various directions for any given type of base enamel:
Grain calibration: obtained by crushing frit scales and subsequent sieving.
This is a simple mechanical operation which allows separating granules say between 400 and 600 microns or 600 and 1000 microns or between 600 and 800 microns if so required.
Coloured frit hardness from which granules are obtained: if the glass building the granule is harder than the base enamel we obtain small "icebergs" emerging from the base enamel. When observed, they look smaller than their real size, when touched, they give a sensation of roughness. The quality of this granite is poor.
If the glass building the granule is considerably softer than the base enamel, we obtain granules which sink and grow in size according to the fusibility difference between the two glasses. In this case, granite quality improves in size and contour clearness.
We recommend using slightly softer granules than the base enamel; if they were considerably softer, they might produce too much dip and ugly looking dimples.
Consistency between base enamel and granule frit: clear contrast between base enamel and granule, or a slight shaded contour can be achieved by acting on this feature.
It is remarkable to consider that for a given granule, a different visual effect of size and of contour line clearness can be obtained, by varying base enamel hardness (figure 5).
b2) Production of coloured granules
The art of colouring glass and therefore frits by adding metallic oxides to the smelting bath is centuries-old know-how. There were different schools in Europe and here in Venice, at Murano not far from here, we can admire the brightnessof glasses coloured by adding metallic oxides or other stains to the batch to be smelted, with ages-old techniques handed from father to son. In our case we must firstly select the base enamel to the above rules as above and then add suitable metallic oxides or other stains to the batch to be smelted, as follows:
Dark colours: one element or a mix of Manganese, Cobalt, Chromium or Iron oxide
Light colours: Zinc, Alumina, Cerium and/or Titanium Oxide.
After thorough mixing, the batch is smelted in suitable smelters. Special colour shades can be obtained with metal salts instead of a percentage of whole metal oxides; a turquoise shaded glass is for instance obtained with copper carbonate. The first approach to reproduce a coloured frit is to use a crucible smelter (50-200 g of glass) - (figure 6).
The use of a rotary smelter is recommended for batches ranging from 10 and 50 K ,while a normal production rotary smelter can be used for bigger 100 / 500 Kg batches. The rotary smelter is the most suitable for optimum batch mixing before and during smelting, as well as for washing its inside between subsequent batches.
Each smelted batch is checked by comparison with a suitable smelting standard - the so-called flow or button test - as well as with an enamelled plate compared with its counterpart applied with a standard and fired under the same production firing cycle as the customer's (figure 7).
Fig. 7 -
Discharge of a rotary smelter
c) Possibility of producing R. & E. enamels - general information
Some operations are transferred to the frit and enamel producers, and the
Customer should not only have the required know-how but also all the machines
normally used by enamelling shops such as mills, classifiers, and magnetic vibrating
sieves suitable to produce the required products in a short time (figura 8).
Example of granite production
C1) The base enamel is first wet milled to standard fineness. Then a sample for a quality control is taken for applying and firing a steel plate. If the result is satisfactory, the sequence of operations continues as follows:
C2) The first granulate - the white one for instance - is added to the base enamel. (This corresponds to the one yielding minimum size after firing ). After starting planned mill rotation, a new sample of the resulting enamel is taken for applying and firing a second plate. If this test too is satisfactory, the procedure continues as at C3). Smaller granule size due to subsequent milling must be taken account of.
C3) The second calibrated addition of granules must be now put into the mill. It is medium size and second colour. The suitable number of mill revolutions is now set and, a further control of an enamelled and fired steel plate is performed at cycle end. If results comply with standards, the process continues with a third addition of calibrated granules.
C4) This corresponds to final largest granule size. Colour is dark brown in this case. All milling and control operations must be repeated as above. In the case of a fourth granule addition, the entire procedure as above should be repeated, otherwise the process runs immediately as follows.
C5) The mill is now discharged and the product flowing through a magnetic vibrating sieve is freed from all paramagnetic particles as well as all kinds of impurities larger than the sieve mesh net opening, which should of course be more than maximum granulate size. The sieved enamel slip can be immediately put into plastic containers (25-50 l) supported by conventional wooden platforms.
C6) Before closing the containers a 100X200 mm steel plate should be applied with a sample of the enamel taken from the mill, than fired with the same cycle and so on as the customer's and, if found satisfactory sent together with the granite enamel.
The slip has a consistency of 6 closed Bayer blade holes so the enameller only has to mix the slip into the container and add one or two glasses of water if required.
It is worth noting that granules ranging in size from 300 to 1000 microns and therefore much more than base enamel particles do not tend to set for a short time even when the slip remains at rest. Normal slip stirring is thus enough to ensure good granule and enamel particle suspension (figura 9).
d) Capability to reproduce the firing and application cycle, if required
A small continuous Laboratory or Pilot kiln, flexible, exact, and TIME, SPEED and TEMPERATURE adjustable is needed to guarantee a granite enamel prepared as specified above is suitable to properly reproduce required results: so Laboratory technicians can set up the cycle to ensure the same or at least comparable results in terms of SURFACE, COLOUR - ADHERENCE - MECHANICAL RESISTANCE, to be achieved with firing in the Customer's kiln. The cycle should also be susceptible of adjustment to new operating conditions.
Application is normally done by spraying 1 coat + 1 fire on decarburised, Niflashed sheet steel, with 3 colour and size granules.
If required, a "combi" application can be used for the few cases where required by sheet steel quality and circumstances, on a previously fire denamel coat.
e) Availability of experienced enamelling technicians
The various preparation steps of these products must be completed by experienced enamelling technicians, when our plant is asked to prepare an easily applied ready to use enamel without set-up problems.
They must have the overall knowledge and the operating mental attitude of traditional enamelling shop masters, increasingly less available these days. Their responsibility is to develop enamel with production reality in mind, check the granite with the Customer and set up control tests to guarantee quality reproducibility.
f) Capability of technicians to train customer operators (if required)
To complete the presentation at points d) and e), assistance to Customer personnel is sometime needed to present innovative application techniques, often required for starting off Ready & Easy production.
Conclusions on polycrome "Graniti"
To conclude, the R.&E. enamels polychrome granites shortly presented so far represent the possibility of completing the range of products the market wants when a "natural stone look" is required.
These enamels have been designed for aesthetically matching ovens and cooking plates or ranges with synthetic polymer washbasins with a granite effect. The best possible improvement would of course be reached if plastic washbasins were replaced by press steel ones with a Ready & Easy polychrome granite, given the higher technological and hygienic performance of vitreous enamels.
We would therefore like to remind all marketing experts, engineers and architects that granite enamels mean you can achieve all imitation natural stones obtained with synthetic polymers for work planes and washbasins.
The decoration possibilities of vitreous enamels are many more however.
Last but not least, an enamelled washbasin is a peerless item in terms of hygiene, colour durability, fire resistance, does not release fumes or toxic compounds and is abrasion and corrosion resistant.
When impacted, it might slightly spall but never break into pieces as a ceramic or plastic item would.
So we recommend considering an porcelain enamelled washbasin when selecting a new item (figure 10).
Metallizing effect: copper - bronze - gold
The attempt to give to the porcelain enamelled surfaces a metal like look is not new, but results were not altogether satisfactory when it had to retain this appearance with ecologically safe materials.
The product line of Ready & Easy metalising enamels in their copper, bronze and gold hues represents a successful achievement in terms of aesthetics and the absence of harmful/toxic component release.
The sector of kitchen ranges and wall ovens, and even washbasins and stove fume tubes is a special niche demanding a metal reflectance enamel with a copper or bronze/gold appearance. Demand is so pressing in some cases that, in the lack of suitable enamels, some fell back to using metalising paints and giving up special features of vitreous enamels, namely heat and detergent abrasion resistance, hygiene and durability. Just imagine an oven fume exhaust pipe painted with a copper like varnish.
These pipes do unfortunately exist, but only because there is no enamelled item. The same request could not be filled for cooking plates, though enamelling factories has often asked us to supply a copper, bronze or gold appearance vitreous enamel.
Our technical staff at in Dalmine received the request, designed a special project and developed an R.&E. enamel with the required metal appearance.
Project: "Ready & Easy" metallising enamels
a) Pigment research
The market offers metalising pigments which are very good under very low temperature firing conditions but are too unstable at vitreous enamel firing temperature ranges. They are also very expensive.
An excellent metalising pigment particularly stable at the operating conditions we needed was found in Wendel's large stock.
Practical tests were completed to characterise the enamel mix we needed to guarantee colour stability and surface finishing.
It was a rather long search and set up, as only few frits simultaneously can feature adherence, mat, semi-mat and glossy surface added to metallizing pigment properties.
We had to consider that a shaded zone between dark and dark brown and the metalising pigment was required for the rustic variety (figure 12).
b) Accettable Cost
To offset this technical issue with acceptable cost we concentrated on an enamel to be applied with the 2 coat + 1 firing technique. The first coat provides adherence, develops background colour, characterises the kind of surface and facilitates application of the second coat. The second coat essentially develops metal reflectance by protecting the pigment during firing.
It does moreover retain the kind of the surface required. This coat contains a particularly high concentration of metalising pigment, so it should be applied at a thickness of few microns (in enameller slang "half a coat"). With reference to application weight, the cover coat should be 20% of the base coat (figure 13).
c) Easy application and results
Both the base enamel and finish metalising coats are prepared at our Dalmine Unit, where they are tested in the same operating conditions as the Customer's, where only slip consistency control and application are required, easy tasks to complete.
A few hours of training to customer operator personnel is enough for this. The result is guaranteed provided directions are followed (figure 14).
Thanks to "R.&E." metalising enamels, the market can now offer cooking planes, smoke exhaust pipes, furniture and rustic ovens, where surfaces have true metal reflectance with copper, bronze and gold-like appearance, as requested.
We are glad to state we could complete metalising enamelling tests on cast iron items with excellent first class results, so the market so if demands we can supply metalising R.&.E. for cast iron items as well, in copper, bronze and gold, mat, rustic or half-glossy finishing hues (figure 15).
Reduced bubble structure enamels
These are of vitreous enamels for special applications and their common features are a reduced bubble structure. Their composition consists almost exclusively of frits prepared by smelting and maturing or refining the batch of raw material in rotary smelters. At process end, the smelted batch is released through the outlet-inlet hole, and laminated with a special roller mill whose rolls are internally cooled with cold running water. The band of frit obtained is then broken by special crushing rolls into scales with a reduced and uniform bubblestructure ,
the same is true for the enamels obtained from the same frits.
Some examples of their application are the "R. & E." products with low and uniform bubble structure used for encapsulating Ni-Cr resistors or the chemicals-resistant enamels for multi-layer high-thickness enamelling of chemical and pharmaceutical industry apparatuses.
a) Multi-layer enamels for ceramic resistors
Ceramic resistor producers use a 1¸ 1.5 mm total thickness multi-layer enamel obtained by multi-layer application and multiple firing.
This enamel should encapsulate resistor metal wire and laminas by wetting them to adhere perfectly to the metal or ceramic support.
The purpose of this operation is to insulate, protect and disperse heat. After finishing the enamel layer of the resistors, it must be free of crazing hairlines or bubbles, which represent reduced electrical insulation, or an entry point for corrosion as well as a starting point for unwanted crazing.
Porcelain enamel has a low price-competitor in this case too. This does not apply to plastics or varnish but cement, which, though lower quality than enamel, is easier to apply.
This sector that once only demanded frit now also requires "R. & E." enamel too that can satisfy the requirements described.
b) Chemical- resistant multilayer enamels
The producers of enamelled pressurized vessels for the chemical and pharmaceutical industries need a multi-layer high thickness enamel featuring good mechanical stress resistance to shock and the like, as well as chemical resistance, but absolutely free of bubbles, unsmelted particles or hairlines.
No defect can be accepted due to the value of the enamelled item and to the use to which the pressurized wesser will be assigned. These customers normally only request frits that are sometimes applied dry as received and are always looking for additives and mill formulae in order to obtain a reliable enamel free of all the defects mentioned above.
c) How to reduce the bubble structure and unfused particles
Reduction of the bubble structure and unsmelted particles is obviously a quality improvement also for normally used enamels but is not controlled because it is taken for granted that the optimum manufacturing process, used by the most qualified frit producers, gives a very low amount of these defects.
Moreover for the normal industrial enamelling the problem has no importance.
However, with the special requirement of these enamellers in mind, we concentrated on thoroughly controlling for the presence of bubbles and unsmelted particles in the frit.
These enamels are wet milled with a minimum amount of mill additions, so bubble structure reduction should be achieved mainly in the frit (figure 17).
Our experience with today's technically advanced and microprocessorcontrolled rotary smelters taught us to use simple testing before discharging the smelted batch.
The first is the old test of the draught of an enamel thread from the smelted bath; it should be about half a meter long and present the smallest possible diameter.
If smelting is completed or mature, no knot of infused material should be felt between two fingers running along the solidified thread.
The second test is the so called caramellone test, which consists in taking a sample of smelted and mature transparent frit from the rotary smelter with a 10x10x2cm steel mould. From the cooled, transparent sample one can identify whether the batch is free of unsmelted particles; when no bubbles are visible, they might only appear after frit remelting at the enamel firing temperature, that is to say at customer operating conditions (figure 18).
Italian enamelled resistor or enamelled pressurised vessel producers asked
that their frits be smelted in rotary smelters to ensure complete smelting.
They also recommended extended smelting time for complete gas release.
The study of bubble and unsmelted particle reduction in the frit started jointly
with the most demanding customers at that time.
The first test consisted in remelting a frit in a small 300 g crucible with an
electric muffle kiln heated to 850 - 900 °C and left to slowly cool to avoid
inducing internal stress into the enamel, and thus spalling or crazing.
Simple visual inspection is enough to check for the absence of bubbles and
unsmelted matter (figure 19).
d) Simplified method for bubble structure control
A simpler and more reliable method for assessing bubble rate and the
absence of unsmelted matter was then sought for; Today's situation is as
Results of the search
Neglecting ceramic crucibles was first considered to prevent possible
interaction between the refractory and the smelted frit.
The attempt was then made to give the resmelted frit sample an easily
observed optimised diameter and thickness shape(figure 21).
Bubble and unsmelted matter reproducibility was then
temperature and thickness parameters (figure 22).
Preparation of a 100x200 mm flat refractory plate coated with a thin easy to remove powder-layer .
Preparation of refractory 60 mm diameter rings cut from a refractory pipe, also coated internally with the easy to remove powder-layer.
After introducing 50 g of frit inside these rings, each ring is covered with another refractory plate, then all rings are introduced into the muffled kiln (figure 22b).
Finally, temperature and total permanence time, such as 800¸ 850 °C x 30,
60 min. for instance, are fixed, after which all rings are left to cool inside
the furnace, so no crazing appears at the lens of melted frit.
The following can be achieved by proceeding as above, from the "lenses" or
transparent regular discs, without polishing or buffing:Finally, temperature and total permanence time, such as 800¸ 850 °C x 30,
60 min. for instance, are fixed, after which all rings are left to cool inside
the furnace, so no crazing appears at the lens of melted frit.
The following can be achieved by proceeding as above, from the "lenses" or
transparent regular discs, without polishing or buffing:
FTo compare frits smelted with different production methods or cycles (figure 24).
F To perform a computer image analysis, without further mathematical data processing.
F To prove the amount of bubbles can be reduced by using lamination rolls for quenching smelted frits and their enamels.
The frits for the special enamels considered in this paper are normally smelted in rotary smelters. This technology enables obtaining as mature as possible smelting. The visual inspection of the lenses obtained by frit resmelting shows that various products offered by the market can be reviewed.
It also shows that further bubble and unsmelted particle reduction can be achieved by quenching between cooled rolls at the rotary smelter discharge step, and that normal product enamels too can present the same advantages if so desired.
Ready & Easy decorating pastes for special applications
Many enamelled items need a decoration to offer information close to adjustment knobs or to identify the trade mark or the manufacturer, and sometimes for pure decoration (figure 25).
This paper only covers products requiring special application and firing as separate from traditional products. Decorating pastes are normally applied by silk-screen printing and fired at 780-800 °C.
They consist of a glassy matrix (>80%), colour pigments (<20%) and a small percentage of suspending and dispersing agents with an oil vehicle to enables silk screen printing application.
Old products are still used, added to more recently developed watercompatible silk-screening vehicles, assessed as being more eco-compatibles than the formerly used pine oil.
Only the more qualified frit and enamels producers can supply good decorating products, guaranteed for results and the use of non-toxic and environmental friendly ingredients.
New realities and requirements
Many Italian enamellers have chosen to fire their items with the kiln adjusted to 840-850 °C. This means doing without silk screen printed items until kiln temperature is lowered to 780-800 °C to fire them.
The above involves a series of problems ranging from late parts assembly to lost working area and segregating items, as well as quality loss due to rejects and repairs, dirt and dust pollution as well as possible shock.
Frequent kiln temperature changes lead to obvious problems of lost time and quality, waiting for the kiln to settle.
a) Pastes for firing at 840 °C
Some enamelling factories have chosen not to change the kiln temperature at all, at the very most only the conveyor belt speed is slightly modified. So these factories have same firing curve, and asked us to provide a silk-screen product to be fired at higher than usual temperature, namely 840 °C.
Conventional pastes fired at this high temperature react with the substrate and sometimes sink, and change colour, in other words need to be re-formulated. Hardening their vitreous matrix is not enough; we must change it with a suitable one and sometimes also change the colouring pigment type.
Another obstacle is colour change some white or light enamels are subject to after a second firing.
The solution of the problem is relatively simple and consists in selecting an enamel scantily sensitive to temperature variations and designing the items to be decorated with shape or geometry hiding rather than stressing possible slight colour differences with the adjacent item.
To guarantee the constance of colour and sharpness of contour, in the absence of defects, the decoration surface can look slightly mat. This visual appearance is however normally of little importance for small decorations as it is hardly perceived (figure 25).
b) Pastes for firing at 600 °C
Some enamellers also have a plan for firing decorations in a small kiln.
It requires the smallest possible, guarantees good decoration results, simplifies technical-aesthetical problems, and minimises rejects and repairs as well as management costs.
A silk-screen printing operator can put the item straight onto the conveyor belt, roller chain or collecting belt of a small kiln designed for this purpose.
The advantages are:
immediate decorated item firing, so they are immediately available to the assembly line.
elimination of downtime and other operations causing rejects and repairs.
brilliant colour and surface of decoration at 600 °C (acid resistance= AA).
reduced kiln and labour operating costs.
no colour variation to the fired base enamel.
Successful practical factory-trials were completed for decorations or to prove that low temperature re-fired enamel is not subject to some feared defects.
c) Pastes for tampografic application
Cooking planes and oven-doors are the enamelled parts of the range featuring an increasingly attractive and ergonomic look.
This can involve a shape and a geometry having such relief as to make that silk-screen printing impossible. Decalcography must be eliminated in these case because of its special labour costs.
The solution is relatively easy for range producers: buy a machine enabling decoration application with a silicone mould. We shall not now describe this machine, which ceramists and enamellers have been using for a long time, but we proceeded by facing the difficulties - this time charged to the enamel producer.
We first developed a tampographic vehicle also featuring the capability of being caught and transferred to an inclined or relief plane in addition to presenting the properties of a silk screen vehicle.
The solution to the problem was found with experience, innovative products and, joint supplier-customer efforts.
d) Conclusion, "Ready & Easy" decorating pastes
The above decorating products are completely innovative in terms of their vitreous matrix and colouring pigments, and in some cases also in terms of the application vehicle. As decoration pastes, these products were born ready touse, which means really Ready. They are also EASY, as the producer takes care of all problems to overcome all application difficulties with innovative products and their optimisation.
This is why we feel that our decoration pastes can be called Ready & Easy decoration pastes.
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