Magnesium carbonate

I know inks are different, but GENERALLY how much magnesium carbonate do you use for fairly fluid rubber-based ink as a percentage of the volume of ink?

I’m looking for a general starting point.

I read that to test for the right tack if you put a dab between your thumb and index finger it should stretch about 5/8”. Does that seem about right?

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With over 59 years on-the-job experience as a letterpress machinist I can honestly state that I have never used the stuff to adjust the tack of an ink, either oil-based or rubber.

Work your ink with a palette knife as you would when mixing colours, and so reduce its viscosity before putting it on the press.

Get the machine warm, run it for 10 minutes with a thin film of ink without impression.


Paul, I would like to remind you of your post on
26 oct 09. best james

I use magnesium carbonate all the time in runny litho inks (which are oil-, not rubber-based). For my purposes, on a Vandercook, what seems to work best is to add enough magnesium carbonate so that when you lift some ink off the slab with your ink knife, it drops in a thread such that you can write your name with the thread and it will remain readable for a couple of seconds.



As far as I know magnesium carbonate is not an ingredient in ink, as pointed out, but that was not the OP’s question. Milled magnesium carbonate is a common additive to ink and is sold as such by most printmaking suppliers. It will thicken the ink and alter the viscosity.

Ink treated as such is best for slow moving presses, such as a Vandercook. And, for text work, Barbara’s suggestion is pretty much on the mark. I’d suggest that a further test would be that it hold firm on the ink knife, without the slightest hint of drooling. For platen presses though, you would need a looser ink, and if magnesium carbonate is used as a stiffener, you might need to experiment a bit.



Paul, your such a powerhouse of info. I looked in the
Tamarind Book of Lithography,and found a dozen or so references on “mag” as I have heard it called by oldtimers.
I also looked in the book Printing Inks by Carleton Ellis 1940 and found about the same there. So, I feel your bold statement Magnesium Carbonate is not used in inks is
pardon the pun “tacky”,best ajmes


My UPS guy likes Gans, they pack their boxes with Tootsie Rolls, he waits around for me to open the shipment. I don’t like the fact that they put dryer in their inks though, especially when you don’t ask for it.

A cheap alternative to mag carb is corn starch, which you can buy in the department store. I know a few printers who use it and are quite happy. Barbarian Press told me that they use it. I’d think it not quite milled but to each his own.



I try to buy the inks which work well without modification for the printing I do, but sometimes have used whiting (calcium carbonate) for shortening ink.

We use a product called Cabosil for shortening screen printing inks when printing fine detail. It makes the ink split more readily so that the image comes out very sharply defined. It also serves to increase the viscosity of the inks (resistance to flow). This may be the effect those of you who are modifying inks are trying to get. The Cabosil is available from many sources. We purchase it from a screen ink supplier in 5 gal. buckets (which weigh very little more than the bucket itself).I have seen it advertised as an additive for increasing the viscosity of epoxy resins, too. The Cabosil is very finely milled silicon dioxide, and is not the easiest stuff to handle. It looks like fluffy snow. As with all fine powders, it should be handled with a particulate mask, but does a great job of modifying the inks for our use. I think it would do the same for letterpress applications as it does for screen. The old books list calcium carbonate or whiting (and I did find some suggesting magnesium carbonate), but there’s no reason newer alternatives would not work well if required. Whatever you use, work it well into the ink before using it on the press.