Frequently Asked Questions About Pottery and Glazes
Here you will find helpful information about clay, engobes, underglazes and ceramic glazes, as well as firing temperatures, glaze application, coat thickness, glaze defects and the decoration of pottery and tableware.
An engobe is mainly made from liquid clay and is used to add colour to pottery. It primarily changes the colour of the clay but does not normally form a sealed, glass-like surface.
Underglazes are particularly suitable for fine drawings, lettering, patterns and coloured areas. They are often covered with a transparent glaze to create a sealed surface that is easier to clean.
A ceramic glaze melts during firing and forms a glass-like surface. Depending on the product, this surface can be transparent, coloured, glossy, matt, textured or feature special effects.
Whether an engobe or underglaze should also be covered with a transparent glaze depends on the desired result and the manufacturer’s instructions.
Every glaze has a specified firing range. This range must be suitable for both the clay being used and the intended firing temperature.
Earthenware glazes are fired at lower temperatures than stoneware glazes. Stoneware glazes are intended for high-temperature firing.
If a glaze is used outside its recommended firing range, its colour, surface, adhesion and flowing behaviour may change considerably.
BOTZ PRO has a particularly broad firing range and can be used for both earthenware and stoneware. Nevertheless, always check the information provided for the individual colour.
Mayco Stroke & Coat can also be used across a broad firing range. However, during high-temperature firing, the colours may change or appear paler.
The firing results for individual Mayco Stroke & Coat colours at low and high temperatures can be compared on the Mayco website.
View Mayco Stroke & Coat firing results
Our tip: Before purchasing a glaze, always check its firing range and compare it with the firing range of your clay. We recommend carrying out a test firing whenever you use a new combination.
No. The clay and glaze must be compatible in terms of firing temperature and thermal expansion.
A glaze may look very different on red or iron-rich clay than it does on white clay. Transparent glazes may, for example, appear darker, warmer, slightly coloured or cloudy.
If the clay and glaze are not compatible, problems such as crazing, flaking, bubbles or other surface defects may occur.
Our tip: Test a new combination on a small sample made from the same clay. Fire the test piece using the same firing programme that will be used for the finished item.
Most ceramic glazes are applied to a piece that has already been bisque fired.
The first firing makes the piece stable while leaving it porous enough to absorb the glaze.
Engobes can often be applied directly to leather-hard clay. Depending on the product, they may also be suitable for dry or bisque-fired clay.
Some underglazes and decorative colours can also be applied to unfired clay. Always follow the application instructions provided by the manufacturer.
The required number of coats depends on the glaze, the application method and the desired surface.
Ready-to-use brush-on glazes often require two or three even coats. However, the instructions provided by the manufacturer for the individual glaze should always be followed.
Allow each coat to dry sufficiently before applying the next one.
A coat that is too thin may appear patchy, pale or incomplete. A coat that is too thick may run, bubble, flake off or melt onto the kiln shelf.
Our experience: Several even coats often produce more reliable results than one very thick coat.
Stir the glaze thoroughly before applying it so that any settled ingredients are evenly redistributed.
Before glazing, the piece must be:
- clean
- dry
- free from dust
- free from grease
Dust, grease from your hands or other residues can prevent the glaze from adhering properly and may cause bare patches after firing.
Apply brush-on glazes as evenly as possible using a suitable, soft glaze brush. Avoid repeatedly brushing over areas that have already started to dry.
The base and all surfaces that touch the kiln shelf must remain completely free from glaze. Otherwise, the piece may melt onto the kiln shelf during firing.
When dipping a piece or carrying out more detailed glazing work, Wax Resist can be used to protect specific areas, such as the base, from the glaze.
The wax creates a barrier that causes the glaze to bead and run off. The wax burns away during firing. Any drops of glaze that remain on the wax should be carefully removed before firing.
The glaze should be completely dry before the piece is placed in the kiln.
The required drying time depends on factors such as the thickness of the glaze, room temperature, humidity and the size of the piece.
A freshly glazed piece may already appear dry on the surface even though moisture is still present inside the ceramic body.
Allow the piece to dry for a sufficient amount of time in a well-ventilated area. Avoid drying it too quickly or unevenly.
Many glazes can be combined or applied in several layers. This can create new colours, textures and flowing effects.
It is important that the glazes being combined are suitable for the same firing range. Glazes intended for significantly different firing temperatures should not be combined without first carrying out a test.
Applying several glazes on top of one another increases the total glaze thickness. This may cause the glaze to flow more strongly during firing and, in the worst case, drip onto the kiln shelf.
Test new combinations on a sample tile made from the same clay. Make a note of:
- the glazes used
- the order in which they were applied
- the number of coats
- the clay used
- the firing temperature and firing programme
We recommend applying a suitable kiln wash to protect the kiln shelves. Kiln wash is applied to the kiln shelf and can help limit damage if a glaze runs unexpectedly.
Even when kiln wash is used, strongly flowing glazes should not be applied directly down to the bottom edge of the piece.
Many glazes only develop their final colour and surface during firing. The unfired colour of the glaze in the container may therefore look very different from the finished firing result.
The result is influenced by factors including:
- the colour and composition of the clay
- the firing temperature
- the hold or soak time
- the thickness of the glaze
- the firing programme
- the position of the piece inside the kiln
- glazes applied underneath or on top
Manufacturers’ colour samples should therefore be regarded as a guide. The actual result may vary depending on the clay, application and firing conditions.
Our tip: When using a new glaze or clay-and-glaze combination, always prepare and label a test piece.
Only use glazes for food and drink tableware that the manufacturer has expressly approved for this purpose.
Even an appropriately labelled glaze must be applied correctly and fired within its specified firing range.
After firing, the surface should be sealed, smooth and free from cracks, bubbles, pinholes, flaking or sharp areas.
Crackle glazes and deliberately cracked, very rough or heavily textured surfaces are often less suitable for areas that come into direct contact with food.
Always follow the manufacturer’s information for the specific glaze, firing range and suitability for food and drink tableware.
Glaze defects can have several different causes. Common reasons include:
- a glaze coat that is too thick or uneven
- dust, grease or dirt on the piece
- glaze that has not dried completely
- an unsuitable firing temperature
- an unsuitable firing programme
- clay and glaze that are not compatible
- gases trapped in the clay or glaze
- a hold time or cooling phase that is too short
Because several causes may occur at the same time, document the clay, glaze, number of coats, firing temperature and firing programme as accurately as possible.
Test pieces can help identify the cause and find a suitable combination for future work.
Ready-to-use brush-on glazes are particularly practical for beginners. They are easy to stir and apply precisely, and only a small quantity of glaze is required.
Beginner-friendly glazes have a clearly documented firing range and relatively stable firing behaviour.
Strongly flowing effect glazes, very thick applications and complex combinations of several glazes generally require more experience.
Our tip: Begin with a well-matched combination of clay, glaze and a clearly defined firing temperature. Prepare test pieces first and record the number of coats and firing programme.
Do you still have questions about clay, engobes, glazes or choosing suitable products? We will be happy to help.
Contact us