Writing information on test tiles

I do a lot of glaze testing, which requires me to write testing info on my small test tiles. When I first started out, I used red iron oxide mixed with water and a paintbrush. This is what my teacher used and it worked well enough, but I was not good at writing with a brush and found I couldn’t get enough info onto my test tiles.

So then I bought a small squeeze bottle with a fine metal tip that I filled with black underglaze.

The only problem with that is sometimes the tip gets clogged and either no underglaze comes out, or a big blob of underglaze comes out and I have to start over. You have to squeeze just the right amount for legible printing. I got fairly good at it and used this method for many years.

Until… I discovered underglaze pencils.

These work exactly like pencil crayons and I could now easily write a lot of details onto a test tile. They work great on smooth, bisque-fired clay. They don’t work so well on textured clay, like a slab with canvas texture pressed into it. My little squeeze bottle worked better for textured clay.

But for writing on my smooth, extruded test tiles, the underglaze pencil is perfect. The only real problem with that is the cost. The Amaco brand underglaze pencils cost like $20 each!

This was not enough of a deterrent for me since they were so useful, but I guarded my underglaze pencil with my life. If you drop it and the “lead” breaks inside, then when you sharpen it the lead just falls out, a couple dollars worth at a time. There goes your $20. (I don’t know what the correct term is for a pencil “lead” that isn’t made of actual lead or graphite…?)

A cheaper alternative?

Recently, a couple of my students shared that they had tested their regular ol’ pencil crayons (apparently some people call these “coloured pencils”?) at cone 6, and some of them survived the firing! We’re talking cheap, Crayola pencil crayons that are used for colouring. I was very excited to hear this! I just so happen to have some Crayola pencil crayons that I’ve probably had for the past 25 years, at least.

I was excited to do some testing.

I then promptly forgot about testing my pencil crayons until I started working with a very dark clay called “Coffee” by Plainsman Clays. I used my black underglaze pencil to write on my test tiles and they came out of the kiln almost blank. I could just barely make out some writing on them.

That’s when I remembered – pencil crayons! How great would it be if I could use a white pencil crayon on my dark clay body?

So I finally did some testing and was very pleased with the results!

Testing pencil crayons at cone 6

These are the pencil crayons I used. Hey look, they are called “coloured pencils”. I’ve been calling them “pencil crayons” my whole life. Is that a Canadian thing?

First, I tested a selection of light coloured pencil crayons on a dark brown clay body – Plainsman M390.

Every colour I tested on the brown clay survived the firing. This was so exciting! I’m not sure what pencil crayons are made of that can survive cone 6 temps, but some Googling reveals they may contain titanium, chalk, talc or even kaolin.

So then I tested some colours on my white clay body.

Only 3 of them worked on the white clay – brown, dark brown and mahogany. These 3 are likely to contain some iron (or manganese?) in the pigment. 8 of them disappeared on the white clay. None of the blues worked on the white clay, which now makes sense since the blues all turned white on the dark clay. So I would guess that some of these that didn’t show up on the white clay will probably show up as white on the darker clay.

After doing these tests, I mindlessly chose to use the dark brown pencil crayon to write on a couple of test tiles, one of which was brown clay and it completely disappeared during the firing. This also makes sense since the colour of the pigment is basically the same as the clay. Luckily it was only one test tile and I had a matching one on white clay right next to it, so I didn’t lose the info.

This was a fun learning experiment and I now have some cheap alternatives to the $20 underglaze pencil I’ve been using to write on my test tiles. And I’m very glad to have lots of options for writing in white on dark clays.

I encourage you to test your own pencil crayons! Results may vary with different brands. Let me know in the comments below if you have had any success with different brands of pencil crayons. Happy testing!

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