Trouble Shooting and Technical Information

1. Toxic metal release

The ceramic industry has been concerned about the dangers of lead and other potentially toxic metals used in ceramic processing for over a hundred years. In a collective international effort, governments and industry have formed many organizations that specifically address the toxic metal issues. Included among these are the US Food and Drug Administration, the World Health Organization, the Society of Glass and Ceramic Decorators, the Lead Industries Association, the -International Standards Organization, and the United States Potters Association. It is not within the scope of this section to cover the vast amount of information available, but rather to help guide you to the proper sources of information.

The Society of Glass and Ceramic Decorators, based in Washington DC (telephone number 202-728-4 132), publishes their “White Paper.” This paper highlights United States legislation related to toxic metal release and content and provides a list of testing facilities. Legal limits are provided in this paper. A good summary of legal limits worldwide can be obtained from British Ceramic Research Limited. Request special publication 121.

Current lead free ceramic color pallets have limited color range and other qualities of appearance. They are also typically less durable and not as tolerant to process variables. Work continues eagerly to improve them. They should not be use on surfaces that contain lead.

We can control and measure the amount of lead that can be dissolved or "leached out" by food acids using proper location, formulation, printing, and firing. Ways to further minimize toxic metal release include use of these materials only in highlight areas and the use of a flux cap coating. Standard tests of such industrial produced products will typically prove acceptable within established limits (
see Table of Limits).

Products not intended for food use, like tiles, lamps, or ceramic baseball cards are not subject to FDA regulation. The Consumer Product Safety Commission "Ban of Lead Containing Paint and Certain Consumer Products Bearing Lead Containing Paint" in their definition of "paint and other similar surface coating materials, specifically excludes "materials which are actually bonded to the substrate, such as by electroplating or ceramic glazing. Fire on decals fall within this category.

2. MSDS information

All decals are shipped with a fact sheet-providing chemical and component hazard communication called a Material Safety Data Sheet (MSDS). It includes identity, physical hazards, health hazards, precautions for safe use, emergency and first aid procedures, date of last revision, name address and phone number of producer or other responsible party. You are required by law to keep these at a central location, accessible to employees.

3. Durability and UV Stability

When properly applied and fired (see "PROPER FIRING"), all decals produced under our trade name Vitricals™, exhibit the extremely high adhesion and durability common to all vitreous markings. The components literally melt and fuse to the surface being decorated. Our lab is equipped to measure this performance using American Society for Testing and Materials (ASTM) procedures, which can simulate repeated dishwasher cycles, as well as analysis of other tests of adhesion and durability.

For the most severe applications and tests of durability, such as hotel-ware, tile, or where very high resistance to chemical and mechanical abrasion is desired, an in-glaze or under-glaze decal should be considered.

In recent years, there has been a trend toward using colors that were designed for glass decoration for certain over-glaze applications. This is done in order to save on fuel costs and increase output. This is commonly seen on coffee mugs, lighting components and souvenir products. Durability is often compromised and we do not recommend this practice for food contact Surfaces.

In a world so full of disposable items, it is nice to know that the products of our industry can span generations. However, many of our customers are concerned about the durability of decorations on glass and ceramic products. Kiln fired on-glaze and in-glaze decals are the most durable and strongest possible markings achievable on glass and ceramic substrates. Fire on ceramic markings, forming both mechanical and chemical ionic bonds with the decorated Surface, are stronger and more durable than any organic marking. The decoration itself is composed of materials harder than steel so they do not easily scratch. You need only compare the standardized durability tests in the organic paint industry to those for the inorganic ceramic and glass decorating industry to realize the relative strength of our markings. For example, scratch and tape pull tests can be used to empirically quantify many acceptable performance levels of organic coatings. But with vitreous markings, acceptable performance levels are indistinguishable by the results Of Such tests, like the results of 1, 2, and 3 mile per hour crash tests oil a 5-MPH bumper. This durability, combined with the ability to economically reproduce fine art or high resolution graphics makes the products and services offered by Philadelphia Decal superior to all those offered in the long history of ceramic and glass decorating. We've come a long way from the ancient method of hand scratching glass with hard mineral scribes.

There are two basic durability tests, which quality vitreous markings: 1. Detergent tests (ASTM C556 and C676), and 2. Abrasion tests (ASTM C501 and C 1027). Defects not seen upon inspection of a test firing such as those caused by incompatible materials, expansion mismatch, and low bond and surface strength will usually be revealed by Such exposure tests. Attributes such as gloss, color, heavy metal extraction, bond strength, and porosity are often compared before and after durability tests are performed.

Companies involved in ISO 9000 and other sophisticated statistical process quality control programs establish minimum acceptable performance levels for products subjected to Such tests. In support of our products, Philadelphia Decal can help you establish your own test program and can offer Customized testing services for your ware.

Stability of Vitricals™ to UV light
The physical and chemical nature of our Vitricals™ is such that they are not sensitive to UV radiation. The materials, which compose the fired decal, form a matrix of inorganic glass and mineral pigments. These are not like organic coatings, which degrade from UV radiation. Once fired, they have properties more like rock and stone rather than like paint. There have been no documented physicals or chemical property failure attributed to ultraviolet exposure. These types of markings can be considered "UV stable."

4. Removal of decals

The same attributes, which contribute to the extraordinary durability of vitreous markings, make removal extremely difficult. The only known methods are to actually remove the surface either by sand blasting, acid etching or grinding and polishing. It is rarely worth the effort, except for precious metals.

5. Tin vs. airside (decals for float glass)

Visitors to historic Williamsburg Virginia have been told that the glass windows on the old buildings are thicker at the bottom because the flat glass originally installed is a liquid, which has flowed over the years. But, as scientist, we learn otherwise: Glass does not flow below its transition temperature and, as a matter of fact, at room temperatures iron and aluminum flow more than glass. When the window panes for historic Williamsburg were produced, the pane maker dipped a rod in the molten solution and withdrew a glass sheet vertically and allowed it to cool. Gravity made the trailing end thicker and the installer used this natural stability to steady the pane as it was positioned. With the industrial revolution came new ways to make flat glass. However, float glass, introduced in 1959 and manufactured by floating the molten glass on a pool of molten tin, is now the most widely used method of producing soda lime flat glass. So what does this have to do with decals? The surfaces of float -lass have the potential to interact differently when used as a substrate for a fired decal. The tin side of the glass can shift certain colors or even create pits. Therefore, the air side should be chosen for decal decoration. Fortunately, the Molecularly thin tin side of float glass call be detected with a short wave ultraviolet mineral light causing the illuminated till side to glow a white haze. If You are firing decals oil soda lime float glass, test fire both sides. If necessary, obtain a mineral light and mark inventory with a clean burning wax pencil.

6. Firing

"The Ideal Decal Firing"
The goal in firing glass or ceramic (vitreous) decals is to fuse printed inorganic pigment with flux to your ware so as to achieve the desired appearance and physical properties. Firing affects color, surface appearance, adhesion, detergent durability, toxic metal release, and abrasion resistance. Our decals contain organics such as printing oils, and cover-coat which are designed to vaporize and combust ash free during the firing process. Water slide decals carry a water starch layer during application that must be thoroughly dry before firing. Otherwise, steam will lift and pit the decal's surface during firing. Heat release or pad transfer decals use a special waxy hot melt adhesive, which can be fired immediately after application. When decals are applied, it is important that the ,graphical portion of a decal does not overlap the cover-coat edge of another decal, otherwise the burning cover-coat will pit the decoration above it. The organic materials burn away in the range of 150-480°C (300-900°F). Ventilation is critical in this range. If the firing is too rapid and the organics do not have sufficient oxygen, pits can form in the decoration. Vents with radiant shields should be open on the top and bottom of the firing chamber to allow convection airflows to occur. Once the organics have combusted, only the powdered flux and pigment remain. At the peak firing temperature. These powders sinter, melt and fuse to the ware. If the recommended peak temperature is not reached, gloss and colors may not fully develop and the decal may rub off easily. If over-fired, colors may "burn out" shifting toward a green, gray, clear or diffused color. Some pigments have a firing range as narrow as 70°F. Proper firing techniques must also consider firing time. Some cycles, such those for tempered glass are as short -,it 6 minutes, while others are several hours. We fire Most Of Our on glaze decals to peak temperature in 30-60 minutes for glassware and 45 to 90 minutes for ceramic ware. Soak times at peal-, temperature are from I to 3 minutes per millimeter of maximum thickness. Kiln dust and decal water contamination is two additional sources of fired defects. Its important to monitor each step in the application and firing process to assure quality product forms your investment.

Firing Defects
Now that we understand the mechanisms of proper firing (See previous section), its helpful to understand defects which can occur with the use of decals. Defects can occur from a variety of causes: The formulation and printing of the decal, the ware itself, the application process, and the firing process. We have produced a two part video series titled "The Thermal Dynamics of Decal Firing"; the first video shows the proper firing process (video tape of actual surfaces during the firing process) while the second one shows typical defects.

Since the manufacture of decals is a repetitive printing process, defects from the press tend to be of a repetitive nature. Care Must be taken with multiple steps and repeat images to identify sheet location should a repeating defect occur. It is also possible that the inorganic ink components may not be compatible with the ware or firing temperature. Such defects will manifest themselves as expansion cracks or chips, powdery or non-glossy surface, and color shift. In typical cases of expansion mismatch, thin decals perform better while thicker ones can fail by cracks, chips, and pits.



Kiln dirt fired into halftone decal

Typical water blowout

Decal fired over same glaze pit

Glaze pit - A trap for dirt and water

Defects from the ware will typically results from surface defects such as deep pits and bubbles. In some cases, pits can trap dirt and water from normal handling and decoration. Such trappings can blow out during the firing causing regular or irregular shaped pits in the decal surface.

Defects from decoration can occur from a number of causes. The most common are water blowouts resulting from insufficient drying or squeegeeng. Other similar defects can occur from unclean surfaces prior to decoration and overlapping pigment on cover-coat (which can happen with some improperly sized full wrap decals).

Finally, defects, which occur from the firing process, can result from poor ventilation, rapid organic burnout, localized or global over or under firing, and poor kiln atmosphere. In these cases typical defects manifest themselves as discoloration, pits, cover-coat haze, and poor shine from precious metals. Defects caused by kiln dirt in the firing chamber can be misleading as it can appear as a cover-coat outline or haze. This is due to the fact that as the cover-coat acts like sticky flypaper attracting and holding dirt as it heats up and burns away. Microscopic examination of other non-decorated areas (especially horizontal surfaces) will often reveal similar contamination though in not nearly the same concentration as on the decal.

Kiln performance
We are often approached with decal defects that arise from poor kiln performance. However a decorator from a large company with on staff furnace engineers called us with a pitting problem on thick colors that turned Out to be the result of a non-functioning burner in the preheat zone of their Lehr, and we are routinely faced with a problem of color burnout likely resulting from over-firing or poorly controlled kiln. Here are a few tips to help manage kiln and Lehr firing systems:

1. Make sure you know the peak firing temperature of your decal and have an accurate and reliable way of determining the peak temperature in your kiln. Remember, some decal frits perform well only when peak fired in a range as narrow as 40°C.
2. Check the position and condition of temperature sensors assuring that they are not too close to the heat source and that they have not burned out (i.e. some thinner "K" thermocouples will begin to read low once fired over 1500°F).
3. Do not over-soak at the peak firing temperature, especially in the upper range of the firing temperature. This is equivalent to over-firing. As a rule of thumb, soak I minute at peak for each millimeter of ware thickness.
4. Periodically audit various positions in the firing chamber to assure a uniform firing of the load. Free standing guard cones are an excellent and inexpensive method to perform these audits (See Orton publication titled "Cones for Ceramics" for more information). Temperature indicating crayons, pellets, and paints are also useful for such audits.
5. Provide top and bottom ventilation during the organic burn-off stage (i.e. temperature less than 900°F). Assure a dust free kiln environment especially during burnout when the decal surface becomes quite sticky.

Temperature Conversions and cone equivalents


Substrates - Firing Ranges
A- Lead Crystal Glass
B- Soda-lime Glass
C- Borosilicate Glass
D- Earthenware
E- Stoneware
F- Bone China
G- Porcelain
H- Inglaze

960-1000°F (510-538°C)
1050-1175°F 565-535°C)
1200-1250°F (649-677°C)
1200-1375°F (649-746°C)
1350-1475°F (732-802°C)
1350-1500°F (732-815°C)
1475-1750°F (802-954°C)
1800-2300°F (982-1260°C)

Temperatures within 1°C can be determined
by measuring the cone bending angle

The three cone system, courtesy of Orton®
An inexpensive way to monitor local firing temperatures

7. Trouble shooting

Much of the information you need to trouble shoot decals is already found in prior text. In this section we have added a few other topics which we hope will complete this guide.

A Decal Laboratory
A decal laboratory Should have the following tools and equipment: a thickness device (caliper or magnetic deltascope), a ventilated test firing kiln, a properly and consistently lighted inspection station, a color reading densitometer, firing cones, distilled water for decorating, decorating tools, a diamond cutting saw, a magnifying device. More elaborate laboratories should have an atomic absorption spectrophotometer for determining lead and cadmium release, a photo documenting microscope, a detergent durability tester, and an abrasion tester.

Thermal expansion of on-glaze decals
We are often asked about the thermal expansion properties of our decals. Rightfully so, because expansion is an important consideration in selecting any glaze or over glaze decoration. Thermal expansion coefficients, in units of x 10-7 per °C, express the length change ratio of a material to a temperature change of one degree Celsius. If the thermal expansions are too different, factors that influence decal failure such as fired thickness must be held at a minimum in order to prevent chipping or crazing. Newton's third law states "For every action, there is opposed an equal reaction..."; We restate the third law for our application, "Within the body of either glaze or decal, the expansive and compressive forces at the interface must oppose stress generated within the body itself". During the firing cycle cooling process, the decal and glaze begin to shrink at different rates in accordance with their thermal expansion characteristics. If the generated stress exceeds the strength of the material or the adhesive bond, the decal will fail. When the decal fails under compression, the decal will chip. When the decal fails under expansion (i.e. excess shrinkage or tension), the decal will craze even from a microscopic scratch. In either
case, the adhesive bond may fail de-laminating the decal from the glaze. Due to its relative thickness, the glaze body will not fail from decal expansion mismatch. With some types of compression failure, the decal may pop off many days after cooling. Ideally, the expansion coefficient of the decal should be lower than that of the glaze. However this is rarely the case. Fortunately the tempered strength of thin on-glaze decorations afford a acceptably wide range
of expansion latitude. For example, typical decal color pallets with expansions of 75x10-7/°C have perform well on ware with expansions as low as 45x10-7/°C provided they are not printed too thick or fired too high. Generally xpansion ratios of +/- 10% are considered acceptable. Bottom line, it is always desirable to know the expansion of glaze when selectin- a decal material. Furthermore, test firings of available color pallets at different thickness on selected ware help alleviate expansion concerns once decorative graphics are selected.

Table of U.S. Heavy Metal Legal Limits
(adapted from publications of The Society of Glass and Ceramic Decorators)



Small Hollow

Large Hollow

Cup Mug


Food Containers

Restrictions (See below)

Federal Standards
FDA Ceramic Limits:

3.0 ppm

2.0 ppm

1.0 ppm

0.5 ppm

0.5 ppm




0.5 ppm

0.5 ppm

0.25 ppm









3.5 ppm




State Standards








CA Proposition 65


0.1 ppm

0.1 ppm

0.1 ppm

0.1 ppm

0.1 ppm


MA Lead Limits

0.2 ppm

0.2 ppm

0.2 ppm

0.2 ppm

0.2 ppm

0.2 ppm


Restriction Notes:
1 -
Test method ASTM C-738.
2 - Applies to each of 6 units tested except flatware which is the average of six units.
3 - Test method ASTM C-927.
4 - Lip and rim area tested is top 20 mm of tumblers, mugs, and cups.
5 - Items not in compliance must be marked "Not for food use" plus hazard description.
A hole must be bored to prevent use of liquids.
6 - Affects Packaging (i.e. One time use disposable items). Bans intentional addition of Lead, Cadmium and other toxins.
States Affected: CT, FL, GA, A IL, ME, MD, MI, MN, NH, NJ, NY, RI, VA, VT, WA, and Wl.
7 - Requires warnings to be posted if products exceeds limits.
8 - Modified test method ASTM C-738, Also applies to porcelain enamel cookware.

Phone: Phone: (215) 493-8449  Fax: (215) 325-0137  
PO BOX 28 Morrisville, Pa 19067  E-mail   2004 MOLFRAN LLC