Engobes

Bowl with strutting bird, Sultanabad ware, Iran - first half of 14th century, 
earthenware with gray engobe and underglaze painting in blue, black, white slip 
(Cincinnati Art Museum) (from Wikipedia)

I used engobes for the first time just recently and really liked both the processing of using of them and the results.  I am still not 100% sure how to define an engobe.  They seem to be a slip with some melting material added but not quite enough melting qualities to call it a glaze... They can be used on leatherhard greenware or bisque. A basic engobe base recipe can have stains added or you can use in conjunction with oxides to produce definition and/or other colours directly on the  work. 

https://digitalfire.com/4sight/education/creating_a_non-glaze_ceramic_slip_or_engobe_97.html

http://www.ceramicstoday.com/articles/041601a.htm

The thing I like about engobes is their WYSIWYG quality - i.e. they look similar in application to what they will look after firing. I have always struggled with glazing because I find it really hard to visualise how the finished surface will look  - and I'm constantly surprised (often not in a good way) by how things turn out. Over the past couple of years, I've used oxides with dolomite bases pretty much exclusively because I know how these will react in the kiln. This has limited my palette quite significantly.  The discovery of engobes via sessions with Wendy has given me another range of colours to play around with. In tandem with this, I want to continue exploring some metallic surfaces that I've been experimenting with - again these use metal oxides more or less exclusively - but more about those later.