Lead-Free Lessons for Glass and Ceramic Decorators
How lead-free inks differ from conventional lead- and cadmium-bearing formulations
In the world of commercial glass and ceramic decorating, few matters have drawn quite as much attention recently as the issue of lead-based inks. In the last few years, a whole new set of lead-related state and federal rules have surfaced. This has led to extensive research and expense among printing shops and has sparked renewed industry focus on lead-free inks for several applications and technologies--especially those involving products that will come in direct contact with food and beverages. Today, a variety of lead-free enamels are emerging as manufacturers attempt to comply with the new health and environmental standards.
Decorators concerned with lead-release issues typically turn to The Society of Glass and Ceramic Decorators (SGCD), Washington, DC, for support. As recently as March 31, 1999, the SGCD and its members adopted a voluntary standard for lip-and-rim leaching limits. The lip-and-rim area is considered the portion of a glass tumbler or ceramic mug within the top 20 mm of the outside of the item--the portion of the product that a person's lips might touch while consuming a beverage. For these areas, the voluntary standards are now 4 parts per million (ppm) for lead leaching and 0.4 ppm for cadmium (for the previous twenty years, the standards were 25 ppm for lead and 1.75 ppm for cadmium).
Tougher standards exist for areas in which food will come in direct contact with the decoration, such the interior of a mug or the surface of a plate. As of 1996, the US Food and Drug Administration's (FDA) Compliance Policy Guide set the leaching standards for food-contact surfaces from 0.5-3.0 ppm for lead and 0.25-0.5 ppm for cadmium, depending on the specific piece and its function. The Canadian government is working to harmonize its standards with those of the US FDA.
New testing methods to determine lead release are based on many international standards such as ISO 6486/1, ASTM C-738-94 (1994), and ASTM C-927-80 (1993). When considering a decorative application that could place the image in direct contact with a food or beverage, or in the lip-and-rim area of decorated drinkware, it's important to consider all leaching standards and test accordingly to determine compliance.
On both sides of the border, researchers are reevaluating what were thought to be safe standards for lead-release levels. Recent testing indicates that previous lead-level standards were too high, posing a particular health risk to children and pregnant women. Today, where a lead-bearing decoration is used on an item not specifically designed for food use, the product must contain a statement indicating that it is not for food use because it contains lead and cadmium.
In a more radical approach, a 1996 SGCD TechBook proposal suggested that non-food-service items, such as decorator plates, could have a hole drilled in them to render the products unsuitable for food use. Because this would have affected the aesthetic appeal of the product, the industry instead accepted the warning-label option.
As a result of these new standards--and to offset the potential of additional regulations--a range of lead-free glass and ceramic inks have been developed. These formulations satisfy world health and environmental standards, but they've created a series of new production concerns for glass and ceramic decorators.
Examples of lead-free inks
Some of the first lead-free inks developed for commercial applications use bismuth or zinc in place of lead and contain only trace levels of cadmium, if any. Each ink set has unique characteristics when used for glass and ceramic decorating.
Bismuth Bismuth-based ink systems are used primarily for glassware applications, such as automotive, appliance, architectural, and some glass-tableware products. These enamels are usually lead-free and chemically resistant to acid and Calgonite (detergent) testing. However, as with most lead-free enamels, the color palette of bismuth is very limited. Vibrant reds, bright blues, and greens are reduced to less dramatic colors, due to the lack of lead, which usually supports this area of the palette. In addition, firing temperatures may vary from 1125-1292°F (600-700°C), which approaches the sag (softening) point of most glassware items and makes bismuth-based inks suitable only for heavier glass products, not finer stemware or thin-edged glass items.
Zinc For glassware applications, zinc-based ink systems will fire at lower temperatures than bismuth-based systems, usually in the 1070-1170°F (575-630°C) temperature range. Typically, zinc enamels are used in cosmetics, lighting, ad-specialty, automotive, and container applications. Their key advantage over bismuth-based inks is a wider color palette that provides brighter reds, yellows, and greens. However, zinc-based systems lack dishwasher resistance when applied to glass.
In the case of ceramics, the firing temperatures of zinc-based ink systems are much higher, usually around 1470-1652°F (800-900°C). The ceramic enamels are designed to go on top of an existing glaze and have excellent dishwasher resistance.
In most cases, lead-free enamels are produced without cadmium. They are equally stable during normal or fast firing cycles, yielding a higher production rate over a wide firing range than bismuth-based inks. They have excellent resist-ance to acid and alkaline agents, which are major contributors of early breakdown in the dishwasher environments. Nevertheless, they still don't offer the broad palette and durability associated with lead- and cadmium-bearing inks.
The greatest difficulty that screen printers experience with reduced-lead or lead-free enamels is the limited color palette, especially the lack of vibrant reds, bright yellows, and colors derived from these (e.g., orange). These problems result from the absence of lead as a fluxing agent, which restricts color-matching and color-stability qualities in the finished product (Figure 1). Manufacturers continue to pursue alternatives to bolster the range of colors available while maintaining current standards for lead release.
When considering color, the series of enamels used for glassware decorating is quite different from those used in ceramic decorating. The most obvious difference is in the firing temperatures of the inks. Glassware enamels are designed for decorating on soda lime and/or opal glass and are made of particles containing frit and pigments that fuse to the glass surface when brought up to firing temperatures of around 1150°F (600°C). The frit/pigment particle size varies from 5-8 microns (6 microns is average) allowing for the use of a variety of mesh counts. Larger spot-color areas can be decorated with mesh as coarse as 155 threads/in., whereas finer details and halftones are generally printed with a mesh count of about 300 threads/in. In many cases when a job calls for printing very fine detail or process color, printers must remill the pigments to a finer consistency.
Remilling is conducted in a triple-roll mill (Figure 2) that blends the frit, pigments, and carrier medium together as it forces the mixture through a series of three precision-polished rollers. The resulting formulation has a smooth, creamy texture with all particles being evenly dispersed throughout the medium. Ink-milling machines are quite expensive and usually only used by factories producing very detailed, high-quality work. Remilled inks can be printed with mesh counts upwards of 420 threads/in.
Care is required when remilling lead-free enamels--the milling equipment may affect many of the ink's standard properties, including color, gloss, and durability. Companies specializing in fine-detail process- and spot-color transfers typically invested thousands of hours in the research and development of their milling techniques, and most of these techniques are proprietary.
In most cases, decorators will use a premixed ink system purchased directly from their suppliers, and the premixed inks will satisfy most applications. However, if the inks are allowed to sit for a few weeks and the pigments and frit settle, these products will likely have to be reworked with a palette knife or other mixing device.
Controlling color stability is also difficult with lead-free inks, unless consistent procedures are used. A lead-free ink that has been thinned too much may lack color intensity and clarity. And when the ink is too thick, only a small amount of pigment is pushed through the screen while the bulk of the medium used to carry the pigment remains on the screen. The result is a print that appears blotchy or irregular in opacity. For these reasons, it's necessary to strike a balance between viscosity and mesh characteristics to achieve the desired result. Further complicating the color-control issue is that lead-free inks seem to lay down quite heavy during printing, but after firing, the printed image may look weak and flat.
Production Concerns with Lead-Free Inks
The use of lead-free inks has a direct impact on three primary production concerns: firing temperature and soak time, thinning of the ink, and mesh selection. Each of these areas requires special attention in order to avoid unacceptable prints.
Firing temperature and soak time Today, undecorated ceramic products feature a wide assortment of different glazes. Many decorators who produce prints for such "on glaze" applications are turning to off-the-shelf lead-free inks from Cerdec, Johnson-Matthey, and other manufacturers. These inks fire at around 1470-1560°F (800-850°C) and are typically used with porcelain, bone china, vitreous china, and earthenware. This high firing temperature is required to ensure that the ink is fused or bonded strongly enough to guarantee a dishwasher-resistant product. However, the high firing temperature limits the color palette that can be achieved, because the organic ink components necessary to produce vibrant colors (e.g., magenta, cyan, and bright green) would cinder and burn during the firing process.
For this reason, it's important to take the specific color palette of lead-free inks into consideration when initially developing a design. For example, if the job will involve printing on darker base colors, the design should also provide for a white underbase below the image area, and the underbase should fall within 0.007 in. (0.2 mm) of the edges of the image area. This will significantly improve the brightness of the overprinted colors.
Also take care to ensure that the ware itself is lead-free, because lead- and cadmium-free colors can react with a lead-bearing glaze and create some less-than-desirable results. For example, contamination from lead in the glaze during firing can cause a red lead-free ink to take on a dark or brownish cast. Additionally, the firing process can alter the lead-release characteristics of the item.
Firing should be carried out in a well-ventilated static kiln, a pusher-plate continuous-firing kiln, or a lehr (Figure 3) capable of achieving temperatures of 1470-1560°F (800-850°C)--note that most lehrs have a 1200°F (650°C) upper limit. Increasing the temperature too fast can also affect the finished product since all inks contain frit to assist in the flow and bonding of the color to the substrate. If this frit burns off too rapidly, all of the colored pigment will flash off with it before the printed inks on the product reach complete color maturity.
Rapid firing typically leads to designs that look very translucent, with red colors turning to brown and blues changing to a gray-brown color. Also make sure to follow the manufacturers' recommendations for soak time (the time that the ware remains at peak temperature). Generally, it's good practice to test all materials and processing techniques--including firing of the finished piece--prior to beginning production.
Thinning the ink Glass and ceramic inks are thinned by adding more medium to the mixture, and when thinning inks, it's important to keep the ratio of medium to frit/pigment as close to the recommend balance as possible. Adding to much thinner can water down the ink, reducing the concentration of frit required for proper firing. In this situation, the print will take on a flat or matte finish with a lack of color.
In some cases, however, thinning inks may be unavoidable, such as when working with fine-detail applications that require very high mesh counts. Just keep in mind that any time ink is thinned, you sacrifice color and/or gloss to achieve acceptable results.
You may be able to alter an ink's printability in other ways, such as preheating the ink for better flow rates or using alcohol--rather than additional medium--to think the ink. Note, however, that these methods work only for very short runs and a narrow range of applications.
Mesh counts Mesh selection is an essential factor in all glass and ceramic decorating, and the use of lead-free inks doesn't make it any less important. Choosing the wrong mesh can affect the end result much more than in other screen-printed applications because of the importance of achieving the correct ink deposit thickness. As mentioned previously, mesh counts vary from 155-420 threads/in, depending on the amount of detail required.
The most common decorating scenario uses a thread count of 230 threads/in. This count supports a broad range of applications creating a good ink deposit on spot colors and the ability to produce adequate fine detail. Mesh counts above 300 threads/in. are most often used when fine detail is desired. In all cases, when lead-free inks are to be used on screens previously used for leaded products, extreme care should be taken to clean the screens thoroughly prior to the application. Even the introduction of the smallest amount of lead can contaminate and possibly cause the entire batch to fail release testing.
Besides color accuracy, controlling ink-deposit height through mesh selection and press settings is important because an excessive ink deposit may cause the fired imprint to crack or chip off the ware (Figure 4). The maximum acceptable deposit will vary depending on factors such as firing cycle, the body structure and glaze on the product to be decorated, and the composition of the inks involved. Generally, it's best to maintain the thinnest deposit possible. If this leads to a matte or flat finish on test-fired prints, it may be possible to print an additional clear frit overlay, which, in addition to helping with gloss, will provide additional resistance and boost the overall vividness of colors.
Quality of properly cured lead-free inks
A properly cured lead-free deposit will have the appearance of a smooth glossy surface with no visible cracking or chipping--even under magnification. The cure level can be tested by exposing the print to a 3% solution of hydrochloric acid at 68°F (20°C) for five hours. Upon inspection, no signs of acid attack should be visible on the surface. Note, however, that colors such as cobalt blue and orange/red shades have less acid- and alkaline-test resistance than most of the other colors because the lack of lead makes them softer and more porous.
A properly cured product will show good resistance with no visible signs of attack when immersed in a solution of 0.5% of Calgonite solution at 170.6°F (77°C) for 16 hours. Once again, cobalt blue, orange/red, and iron-red may show evidence of a slight deviation in color.
These tests simulate what goes on during normal use with dishwashers etc. and can help predict potential problems in the finished product. In the past, inks were typically rated in terms of dishwashings. But this no longer appears to be the case, because the variety of chemicals used in dishwashing detergents and variety of features in modern dishwashers makes it nearly impossible to establish a standard. Much as the term "water proof" has evolved into "water resistant," dishwasher proof is now "dishwasher resistant." Nevertheless, if proper procedures are followed with emphasis on proper ink deposit and firing technique, a finished piece created with lead-free inks will withstand most anything thrown at it for many years to come.
Among ink manufacturers, you may discover slight differences in the product information provided due to standards that vary by state or country. It is always advisable to contact or research your local governing bodies to determine the standards that apply to your area. Other good sources for help include the technical service departments of various ink manufacturers, the SGCD, and testing laboratories used by the glass and ceramic industry. Regulatory bodies randomly sample glass and ceramic products, and many decorators have found themselves in violation of standards because they didn't take the correct precautions.
Additionally, note that lead-free inks tend to be considerably more costly than conventional inks: While the average cost of lead-based ceramic ink may range from $5-7/lb, and lead-based ink for glassware applications may cost $20/lb, a typical lead-free ink formulation may range from $40-50/lb. This cost has made testing and experimentation prohibitive to all but the most determined or affluent print shops and agencies.
Decorating or imprinting glassware and ceramics with lead-free inks is challenging, but today's technology allows even small- and medium-size facilities to take advantage of these formulations. Keep in mind, however, that the key to successful implementation of these inks is to apply sound screen-printing techniques and carefully research your regulatory obligations and procedural requirements.