Glazes & Glazing FAQ
What is the best way to fire Botz Gold and Reds?
Fire low 1040degC. Leave brick off slightly. Apply thickly.
Is my glaze food safe?
The only way to be sure is to have your glazed pot sent to Ceram Research to have it tested.
Can I use Earthenware brush-on glazes on Stoneware clays?
If stoneware clay is fired to maturing temperature it will be difficult to apply brush-on glazes as the surface will not be porous, if this can be achieved then the earthenware glaze might fire successfully but there is a risk of crazing. The glaze must be fired to its specified temperature. Some stoneware clays are dual purpose and can be used at earthenware temperatures.
What is the difference between a stain and an underglaze & on-glaze colours?
Stains are used to colour clay and glaze and are very strong colours. Underglaze colour is finer ground glaze stain and normally contains a frit or flux addition.
Underglaze colours may also be used to colour glazes and clays but are primarily designed for applying to ware, either greenware or bisque which is subsequently glazed usually with a transparent glaze.They can also be used on top of a white glaze before firing, (Majolica technique).
On-glaze colours are fusible colour where all the constituents are melted together and finely ground, they are applied on top of a fired glaze and are typically fired to 700°C/800°C. They are also called china paints or enamels.
To re-glaze a fired piece you need to do one of the following:
Spray the piece with spray starch, let dry, then reglaze.
Spray the piece with sticky hairspray (usually the cheapest you can find), dry, reglaze.
Heat the piece first, with a heat gun or in the oven or kiln, then apply glaze, (my favourite).
Brush white PVA glue on, let dry, reglaze.
Microwave the piece for 30 seconds. (Some potters say this makes a huge difference, and the piece doesn't need to actually get or stay hot)
Add some suspension agent to the glaze (CMC gum or Bentonite.)
Add some detergent / shampoo to the glaze (baby shampoo is good because it doesn't foam)
To improve your odds further, wash the pot first with ammonia or detergent, wearing rubber gloves, and don't touch it. The oils from your fingers can prevent glaze from sticking.
And... Don't use too much of anything. If you get the coating too thick, you may prevent adhesion instead of encouraging it.
How do I mix a glaze?
Following the recipe, weigh out all the ingredients. Place all the glaze powder in a container at least twice its volume. Add approx. 100ml of water to every 100g of solids.
Leave for 30 minutes to allow the glaze powders to absorb the water. This will break down any lumps and make for easier mixing. The glaze is now in slop form. Mix it thoroughly with a lawn brush, breaking up any large lumps as you go.
Pass the glaze through an 80 mesh sieve into its permanent container, and use the lawn brush to push coarse material through the sieve. This ensures that all the ingredients are of a small particle size and will disperse. Stir the glaze to check its consistency. It should be like single cream, depending of course on your method of application.
Have you ever had a glaze that kept settling to the bottom of your bucket? This is a common problem and may result in firing problems.
When a glaze settles out, some of the heavier components of the glaze settle to the bottom of the container. If you try to use this glaze without thoroughly remixing you will be applying a glaze with key ingredients missing.
A glaze stays in suspension due to the presence of various types of clays, such as bentonite, and/or gums, such as CMC. One common cause of settling out is the addition of too much water to the glaze, which dilutes the effect of the suspending agents and allows some of the glaze ingredients to settle out.
Another possibility is the growth of bacteria which will consume an organic gum, such as CMC, and will lead to loss of suspension.
To prevent bacteria growth do no return used glaze, which has been poured out of the original container, back into the original container.
Also do not introduce possibly contaminated objects, such as brushes, into the original container.
Storing glaze in a hot or sunny environment may also encourage bacteria growth. Freezing can also destroy the action of CMC. And glaze ingredients such as frits, nepheline syenite, soda feldspar and other slightly soluble materials slowly release sodium ions which can deactivate the suspension agent, making it ineffective.
If a glaze has settled out, but has not gone rock hard in the bottom of the container, you can add CMC or bentonite, if you happen to have it.
Calcium Chloride or Magnesium Sulphate are also suspending agents. They would require clay to be present in the glaze mixture, so try adding about 3% bentonite to your glaze. A little trial and error is required to determin the exact amount.
However, you can also use Epsom salts to suspend your glaze. Epsom salts can be readily purchased in most chemists.
First you need to create a saturated solution of Epsom salts by dissolving them in a cup of warm water until no more will dissolve. Then add this solution slowly and carefully to the glaze while continuously stirring the glaze. It should require less than approximately one teaspoon of Epsom salt solution per gallon of glaze.The quantity will depend on the severity of the problem.
If a glaze has got too hard at the bottom to mix back up drain all the liquid off, work on dissolving the solid into the Epsom salt / water mixture, then add the rest of the glaze liquid back in.
Can I mix Glazes?
Yes it is possible to mix say yellow and blue to make green, Its best to test first though!
Do you have a remedy for Pinholing?
When small dots of unglazed or depressed areas appear in the glaze surface, this is called "pinholing". Pinholing occurs when gases in the glaze and clay bubble up to the surface. The gas bubbles pop and a 'hole' appears, which doesn't fuse over.
Fire the glaze higher
Soak the kiln 15minuets at peak temperature to keep the glaze in a liquid stae a little longer.
Wipe all dust off bisque ware
Spray bisque ware lightly with water prior to glazing
For earthenware, a good tip is to ensure that the piece is bisque fired 2 cones hotter than it is glaze fired (i.e. when glaze firing to cone 05, bisque fire to cone 03).
a slower bisque firing cycle to give the carbon more time to burn out;
lowering the glaze firing temperature by 1 cone (for earthenware only); or using a glaze with more flux.
What are the basic ingredients of a glaze
Every glaze is made of the following 3 materials:
1. Silica – Creates glass. Examples: quartz, flint, pure silica
2. Alumina – Stiffens the glaze so it doesn't slide off the clay. Examples: clay (kaolin, ball clay, or fire clay), alumina hydrate
3. Flux – Causes the glaze to melt at a low enough temperature to be used in ceramics. Examples: feldspar, whiting
Whats the differnece between Engobes and Decorating Slips?
Slips are predominantly liquefied clay; they usually are applied on wet to dry greenware. Engobes usually have a lower clay content and also can be used on bisque-fired ware. The word slip generally is used to describe any clay in liquid form. All slips and engobes can be colored with oxides, carbonates and stains. Sometimes very crusty surfaces can be made by applying slips and engobes over the fired glaze surface and then refiring.