Process control: complying with the new lead glaze regulations
10 August 2014
Jez Watson discusses how modern ceramics production methods can be adapted in the context of recent EU legislation concerning the amount of lead allowed in ceramic glazes.
Lead is one the main active ingredients traditionally used in paints and lead-glazed ceramic ware. However, it was not until the end of the last century that glazed ceramic items came under close scrutiny, when an entire family in the USA suffered lead poisoning as the result of using pottery bought from Italy.
As far back as the eighteenth century lead poisoning has been a concern for potters. Staffordshire icon Josiah Wedgwood noted in his memoirs that some glazes reacted with acid fruits and pickles in a potentially dangerous way. He considered them improper and aimed to discover a recipe for making his own glazes with less lead.
In the latter part of the nineteenth century lead poisoning was finally recognised as an illness, with more than 432 cases reported. The following years marked the beginning of legislation and regulation on the admissible quantities of lead used in manufacturing ceramics.
Earlier this year, a new set of EU guidelines further restricting the amount of lead allowed under a maximum Threshold Limit Value (TLV) was introduced. The new directive has sent a shockwave through the industry, as major changes had to be made to manufacturing processes in the name of compliance.
The new piece of legislation demands that: “ceramic articles intended to come into contact with foodstuffs are subject to mandatory provisions for protecting human health, which lay down limits for the extractable quantities of lead and cadmium”.
Furthermore, it illustrates best practice under which equipment, such as furnaces and kilns, should perform and addresses other compliance issues including safety, testing and traceability of recipes.
Industrial kilns, ovens and furnaces are used to finish the glazing on pottery and provide a distinctive sheen on the pattern. They normally offer the user the ability to control the profile for each specific process and load type. This allows manufacturers to control and adjust the intensity of the heat applied to the glazed object.
Controlled profiling is made up of a number of segments called ramps and dwells or soaks that allow the heating process to ramp up at predetermined rates. The temperature is then maintained at specific levels for a set period of time. This stage is called dwell or soak, depending on the load and process curve required.
With most ceramic objects, the temperature profile usually starts firing at a slower rate for the first ramp, from an ambient temperature to 600 degrees Celsius. At this stage, the aim is to remove any moisture from the clay without reaching boiling point. If the temperature is too high and the boiling point is reached, then visible damage such as cracks and uneven glazing will occur.
The next ramp is to gradually increase the temperature to the final set point. Depending on the material used, the first ramp can take anything from four to ten hours to complete, with a 60 to 150 degrees Celsius ramp rate.
The subsequent ramp is mostly fixed around 200 degrees Celsius per hour, with the final top temperature often reaching 1100 degrees. This temperature is then maintained for a relatively short time - ten or fifteen minutes - to complete the firing process.
CD Automation supplies profile controllers that come with free format ramp and soaks. The user can fully customise the profiling with any sequence of ramps, dwells and falls he or she requires. Controllers typically come with up to twenty segments per profile - also called recipes by potters - and allow for up to fifteen profiles to be stored. This feature is particularly important in light of the new legislation and allows manufacturers to trace certain batches and repeat if they were successful.
Other important benefits of the controller include a delayed start, which allows a period of time to elapse before the controller starts a firing. For example, this could be used to take advantage of off-peak power, thus saving money. This can also prove useful when delicate ceramics are being fired or when a special effect is desired.
An event relay output from the controller can be programmed to operate an automatic damper, opening and closing it during the firing. It can also be used at the end of the profile to indicate that the process is complete.
Special functions like waiting are also available. Waiting is particularly useful whenever the process has a delay on the programmed set-point to ensure the load has a ‘guaranteed’ period of temperature or time. Profile recovery is another function, which comes into play after an unexpected power failure to recover the settings of the unit.
The user friendly interface can be tailored to each manufacturer’s needs, from a simple front panel display showing the measured temperature value and target value, with indication of the active segment, to a more sophisticated touch-screen display with graphical representation of the process, containing a data logging facility.
Data logging is encouraged by the new EU directives which ask for stricter traceability and repeatability of recipes.
In addition, thermocouples to measure the temperature, solid state relays to switch the heating elements, chart or graphic recorders for historical records and hand-held thermometers for reference checking can also be used for compliance and quality assurance.
Ceramics manufacturing was one of the first stages of the industrial revolution. Although they are things of beauty, fragile and ephemeral, simple fragments of pots recovered by archaeologists tell stories across the millennia. Today, our industry needs to adapt its practices and create new and safer products.
Recent EU legislation may appear difficult to manage, but manufacturers can confidently use monitoring and controlling equipment to improve their recipes and decrease the lead levels in their products.
Jez Watson is managing director, CD Automation UK
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