July 2024
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Making Printing Ink 1: Boiling Linseed Oil

I have been planning for a while to make my own boiled linseed oil to make printers ink, and I have finally gotten around to it.  Over the weekend, my wonderful husband and two fantastic friends (Thank you Andrew, Laura, and Karl!) and I got together to have a picnic, boil some linseed, and generally see how much trouble we could get into.

Unlike ink used for calligraphy, printers ink needs to be very thick so it sits on top of the type rather than running down into cracks.  It also needs to transfer cleanly from the type to the paper.

Early printers looked to painters rather than calligraphers for inspiration for ink.  There was a varnish used by painters made from boiled linseed oil from the 14th century that is described in “Il Libro dell’ Arte (The Craftsman’s Handbook) by Cennino D’ Andrea Cennini’s.

You ought to know how to make this oil, since it is one of the useful things which you need to understand; for it is used for mordants, and for many purposes. And therefore take one pound, or two, or three, or four, of linseed oil, and put it into a new casserole; and if it is a glazed one, so much the better. Make a little stove; and make a round opening so that this casserole will fit into it exactly, so that the flame cannot come up past it; because the flame would be glad to get to it, and you would jeopardize the oil, and also risk burning down the house. When you have made your stove, start up a moderate fire: for the more gently you make it boil, the better and more perfect it will be. And make it boil down to a half, and it will do. But to make mordants, when it is reduced to a half, put into it one ounce of liquid varnish which is bright and clear for each pound of oil; and this sort of oil is good for mordants.

Boiling linseed oil is tricky and very dangerous.  The boiling point of linseed oil is 316 oC and the auto-ignition point (the temperature at which the oil can combust without an ignition source) is only 343 oC.  This means that if the temperature is not carefully regulated, it can lead to a fire, as I found out.  The vapors coming off hot linseed oil will ignite at 222 oC, so you also have to be very careful to keep the flames far away from the pot.

Early printers would make their own ink.  They would travel out into the countryside with their families, assistants, and apprentices and have a picnic while boiling the oil.  I believe that as the demand for ink and printing grew, there wasn’t time for the process to take up an entire day away from the cities, so the process became more dangerous.

Moxon talks about ink making in his, Mechanick Exercises published 1683.  Although this is post-period, I believe the the sentiments are very period.

The providing of good inck, or rather a good varnish for inck, is none of the least incumbent cares upon our master printers, though custom has made it so here in England; for the process of making inck being as both laborious to the body and, as noysom and ungrateful to the sense, and by several odd accidents, dangerous of firing the place it is made in, our English master printers do generally discharge themselves of that trouble; and instead of having good inck, content themselves that they pay an inck maker for good inck, which may yet be better or worse according to the conscience of the inck maker.

I knew that this was a fairly dangerous experiment, so I took precautions and only attempted to boil a small amount of oil at a time.  I also did  the project at a friend’s house which is also used as our shire’s archery range.  I used an above ground metal firepit for the fire and hung the small cauldron containing the oil from a ladder, which gave some extra flexibility with height and the ability to easily raise and lower the cauldron. For the oil I used food grade Flaxseed Oil (which is what food grade linseed oil is called in the USA).  This product is cold pressed and available at health food stores.  I figured that this would be as close to period linseed oil as I would be able to purchase.  Raw linseed oil available from the hardware store has undergone a very high pressure extraction that destroys some of the components of the oil.

Here is my first attempt to make linseed oil.

First I hung the pot over the fire.  At this point the fire was going pretty well and had not burned down to coals yet.  Additionally, the pot is hanging fairly low.

Linseed Oil Trial 1 Setup

Setup for the first trial of boiling linseed oil

After not too much time, the oil pot started smoking.  Initially, I thought this was possibly water being removed from the oil.  I did not think it had reached the boiling temperature that quickly.

Smoking Linseed Oil - trial 1

The linseed oil quickly started smoking

Moments after it started smoking, the oil caught on fire.  I do not know if the temperature reached the auto-ignition point, if the flames actually got into the pot, or if the vapors ignited and brought the flamed into the pot, but either way the oil was on fire.

Linseed Oil on Fire - trial 1

The oil in the cauldron caught on fire.

Soon after the oil caught fire, it began to foam up and overflowed the cauldron.  This led to an impressive fireball.

Fireball Linseed Oil - trial 1

Once oil foamed over the sides of the cauldron, there was an impressive fireball.

Foaming Linseed Oil - trial 1

A close up of the foaming linseed oi.

I only had a small amount of linseed oil, about a cup, in the cauldron, so it the large flames died down pretty quickly.  The rest of the oil in the cauldron burned off after I had removed the cauldron from the fire.

Linseed Oil Finishing Burning - trial 1

After being removed from the fire, the last of the linseed in the cauldron burned off. I let the cauldron cool for trial 2.

I learned a lot from this trial.  Of most interest to me was how the linseed oil catching on fire led to a large fireball.  I knew from the literature that making boiled linseed oil was dangerous and could cause the place it was made in to burn down, but I wasn’t sure why it would be worse than just the oil burning.  Now I know that once the oil catches, it can quickly lead to a huge fireball.

For the second test, I waiting until I had a good batch of coals and very little flames, and I used an infrared thermometer to check the temperature constantly.  I was also much more careful about the height of the cauldron.

Boiling Linseed Oil - trial 2

The linseed oil was suspended over hot coals, with few flames.

I checked the temperature often and moved the coals around to right underneath the cauldron unless it was boiling vigorously with a high temperature, and then moved the coals away.

Thermometer reading - trial 2

Checking the temperature of the oil (the boiling point is 316 oC)

To pass the time, we toasted some marshmallows around the fire.  We had a second firepit setup, because I knew that there was a chance for problems with the linseed oil, so we had a separate fire for warmth and to cook on, but the linseed oil needed constant checking, so we had to stay close to it.

Roasting Marshmallows

Roasting marshmallows while the linseed oil got up to temperature

Eventually it started boiling.

Boiling Linseed Oil - trial 2

The linseed oil, once it started boiling.

At intervals, I inserted a metal stick into the oil to qualitatively check the viscosity.  It took about 45 minutes once it started boiling to reach the desired viscosity, at which point I removed the cauldron from the fire and let it cool.  At the point that that I decided the oil was done, there was a color change noticeable in the bubbles on the surface.  They had changed from white to yellow.

Final Boiled Linseed Oil - trial 2

The final boiled linseed oil. At this point it was fairly dark out, so I needed to use the flash. Note the yellowish color to the bubbles.

The final boiled linseed oil was a dark nearly opaque yellow/brown.  Before boiling, it had  transparent and a light yellow color.  The color change may be in part from leftover carbon in the pot from the first trial.  I wiped it out, but carbon black is hard to completely remove.  Eventually, I will be adding lamp black to the linseed oil to make printers ink.  I am hoping to make my own lamp black as well.

Final Boiled Linseed Oil in Jar - Trial 2

The final boiled linseed oil. This jar had been put on its side and then put right side up, and you can see that the oil has not returned to level. It is very viscous stuff.

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