Potters who are used to dipping and spraying glazes might be surprised to learn how well glazes can paint on if they have enough gum in the recipe.
Likely you have had problems getting glazes to apply evenly when dipping ware. Many dipping glazes either dry too fast or too slow, drip or curtain during draining, settle out quickly, crack during drying or go on too thick or thin. In industry 'lay-down' is considered a big factor in the ability to fire a piece with an even glaze layer free of defects. On small or delicate ware it can be very difficult to achieve good laydown.
If you have ever used commercial glazes from Duncan or Mayco you know that although the idea of painting glaze onto ware can be quite strange to potters, it actually works very well. It is just about impossible to evenly paint a typical dipping glaze, they dry way too fast and just don't flow like paint. So how does Duncan or Spectrum make a glaze 'paintable'. The answer is gum. Lots of gum.
CMC gum, for example is an organic sodium carboxymethylcellulose (like a glue) that is normally employed to harden unfired ceramic glazes (cement the particles together) for safer handling of the ware. Although CMC gum is not intended as a suspending agent, amazingly it can do exactly that. You can actually make fritted glazes that contain almost zero clay content and suspend and harden them totally using gum.
A very nice side effect of the addition of gum is that glazes dry slower. In fact, you can tune the amount of gum in the mix to achieve the drying speed you want. It should paint and flow nicely but dry fairly quickly after laydown.
How much should you use and how do you put gum in a glaze? Powdered gum resists dispersion in water thus it is difficult to add it to an existing liquid batch. However if gum powder is mixed with other dry ingredients before adding them to the water it can be done (often 0.5-1.5%). A much more effective method is to boil water, add about 25-30 grams of powdered gum per litre and mix vigorously with a mechanical mixer. Normally this mixture is added during mixing to replace part of the water however I have found that for brushing it should be used to makeup the entire water complement. There is room to use 40 grams per liter if needed.
Amazingly, even though the gum solution is quite thick and syrupy, added powder mixes in very easily. The gum solution seems to wet the particle surfaces better than water alone.
Each glaze will paint a little differently. If you find that a glaze dries too quickly and does not flow enough try adding a little more water before deciding that the gum content is too low.
Consider some of the advantages of painting glazes:
If you make small pieces you can make small batches of glaze and even store them in glass containers (i.e. large baby food jars). Thus you can have a lot more glazes at your disposal.
You don't need to bisque fire. Glaze paints onto greenware just fine, you just have to be careful when handling the ware if it is thin.
You can apply very thin layers and apply multiple layers of different types of glaze for visual or decorative effects.
You can make specialized glazes of very low clay content or you can use lower iron and less plastic kaolins (as opposed to dirtier ball clays) to make cleaner and whiter glaze surfaces.
Since CMC is an organic material there can be problems with microbial growth in your glazes. If this occurs, you can use a small amount of Dettol® (from the Pharmacy, actice substance chloroxylenol).