Why pen bleeds through colouring pages, and what stops it

Gsm is grams per square metre: how much a square metre of the paper weighs. It is the easiest way to compare paper. The higher the number, the heavier and usually the thicker the sheet.

Everyday printer paper is about 80 gsm. A good many colouring books use paper not much heavier than that. One Pen Pages use 170 gsm, which is more than twice the weight. Thicker paper has more fibres to hold the ink, so less of it reaches the back. In US books the weight is often given in pounds. The numbers don't convert neatly, so compare gsm where you can.

Even a thick sheet can show a faint mark on the back if you press hard or the pen is very wet. In a book with a picture on both sides of every sheet, that mark lands on the next picture. You spoil one page by colouring the one behind it.

That is why single-sided matters as much as weight. In a single-sided book, the back of each page you colour is blank. If a pen does come through, it lands on white, never on a scene you were saving for tomorrow.

Wet ink travels. Alcohol-based markers, the broad chisel-tip kind, and permanent markers are the usual culprits, because their ink is thin and soaks in. Water-based felt tips can feather a little if you go over a shape twice. Pigment fineliners, with their quick-drying ink, are the least likely to come through.

How hard you press matters too, and so does how long the tip rests in one spot. A light, steady stroke is kinder to the paper. Our guide to the best pens for a one-colour colouring book names some widely sold pens to start with.

You can do this with any colouring book, ours or not, before you start on a picture:

1. Find a blank spot, such as the back of a page or the inside cover.
2. Draw a short line and a small filled square with your pen. Go over the square twice, the way you would fill a shape.
3. Wait a minute, then turn the sheet over.
4. Look at the other side. If you see a faint shadow, that pen is a bit wet for this paper. If you see a dark patch, choose a different pen.
5. Slip a spare sheet behind the page if you want to use that pen anyway.

It takes half a minute, and it is the cheapest way to avoid a ruined page.

Each book has 35 scenes on 170 gsm paper, 7 × 10 inches, with one scene to a sheet and nothing printed on the back. It is spiral-bound, so it lies flat, and it is made to be coloured with a dark felt tip or fineliner of 0.5 to 1 mm.

We don't make claims about every pen on the market, because pens vary. That is why we suggest the test above. If you would like to see how a page is coloured from start to finish, read how to colour a One Pen page.