J_R_Thiele
Stainless
- Joined
- Jan 22, 2003
- Location
- Columbia Missouri
I have been using Charbonnel Aqua wash in Prussian Blue for scraping. Like others, I like the deepness of the color- but find it challenging to apply consistently due to its thickness and tendency to apparently drying and get thicker as the day goes on. I have also found that when I first start to roll it out with a brayer, I often get a large clump of pigment that do not easily roll out with a soft brayer.
Wanting to learn how to make Aqua wash more easily used, I started doing some homework and experimenting.
Paints and inks have been in use since the dawn of civilization. To make oil paint pigments are mixed with an “drying oil”. After application a drying oil reacts with oxygen causing it to thicken and then changes to a solid. Common drying oils include linseed oil, walnut, poppy, and safflower oil, in order of drying speed, fastest to slower. The drying time can be from days to months. Some drying oils are too thick to spread as a paint, so the paint is thinned with a solvent, which evaporates after application.
Charbonnel Aqua wash is a high-quality etching ink made with extra finely ground pigments. It is an emulsion of pigment and a “stand oil” modified to be washable with water. It does not contain water. The stand oil for most of the colors has a viscosity of 30 poise which is considered a medium viscosity. Some blacks also contain 200 poise stand oil, making them more viscous. Pigments can also change the viscosity of the ink. Prussian blue is harder to spread because of the pigment used.
The most important thing I learned from my experimenting is that Aqua wash is not drying during use. It thickens due to change in viscosity after it is applied because it is thixotropic. Thixotropic substances have a time dependent decrease in viscosity when shaken, stressed or shear stressed. The process of rolling out or smearing the ink on to a surface plate decreases the viscosity of the ink film for a short time. In other words, more energy put into spreading the ink leads to a greater decrease in viscosity. We typically will spot an object relatively soon after first applying the ink to the plate and from this get a sense of how thick the ink is. As we scrape, the ink returns to its normal “thicker” viscosity. When we later go back to spot the work again the ink has already been spread so we just work it to even the layer. This is not enough energy to decrease the viscosity, we notice the difference, and assume that it is from drying. To thin the blue on the plate we add our substance of choice (lets pick Windex) - and then mix it in, changing the viscosity by both dilution and initiating thixotropic changes. As we are scraping the Windex is evaporating out of the film on the plate, and the viscosity increases again as the blue is less diluted.
Given the above, our difficulties in getting consistent results are to be expected. This does not mean that Aqua wash is too difficult be used- it just means that we are still learning how to best use it. Richards preferred soft foam roller is going to have a difficult time with undiluted Aqua wash as it is unable to transfer enough energy to decrease the viscosity. Peter holding the brayer at an angle will increase the sheer stress and thus be more efficient at decreasing the viscosity. A harder brayer should also lower the viscosity more than a softer one.
Thixotropic changes are time dependent, meaning that when the shear stress begins, the viscosity decrease takes a consistent time to occur and reach equilibrium. When the shear stress ends the viscosity returns to its baseline state over a consistent time. We need to be aware that the amount of time since we spread the blue may determine its viscosity.
From my limited reading for thixotropic changes to occur requires the substances involved need to be within a certain range of proportions and physical conditions. The same substances in different proportions will not have the same properties and the same substances in the same proportions but different physical conditions will not behave the same. The only sure way to know what a change will do is to try it and see.
In adjusting the viscosity of Canode I usually use surface plate cleaner or lighter fluid, as I have both handy. I have found that the surface plate cleaner, water, and Windex will work for a while, and then you need to add more. I believe this is due to evaporation. The same is true for the lighter fluid and paint thinner- but the need to reapply occurs even faster, as they evaporate faster. When I started using Aqua wash I used the same substances to thin it- and found the same pattern with evaporation- but more variability in outcome.
Both Canode and Aqua wash are emulsions that can be thinned with either water or oil. Using an oil to change the viscosity should work better than using water-based solutions (Windex, surface plate cleaner) or solvents (lighter fluid) as oils evaporate very slowly. This means that whatever effect we get will be stable over time. There is no risk of rust with an oil.
Charbonnel sells the oil they use in Aqua wash separately to use in decreasing its viscosity. At $11 for 75 ml, it costs less than the ink. I have not tried it. It is a “stand oil” which is a type of drying oil.
For scraping, we do not need an oil that dries- even if it does take a week or two to dry to the touch. We need an oil that is less viscous than Aqua wash- and one readily available.
I have started experimenting with corn oil, and peanut oil. To my knowledge neither are considered a drying oil.
The Aqua wash is so thick and sticky that measuring it out by volume is not practical. I elected to measure by weight, using an electronic scale sold for measuring gunpowder. Hint: squeeze the tube to get some blue out, then “cut” it off with a piece of thin wire. The other problem with the stickiness is that it sticks to the side of the container, making it very hard to mix. I eventually heated some piano wire and formed it into a helix at the end. This was put in the chuck of a foredom tool. The helix can be run against the sides and bottom of the container. This worked well.
I first tried corn oil. In the first attempt I put the blue in the cup and gradually added oil until I had a 2:3 corn to blue ratio It was too thick to pour out of the container. Next I put the corn oil in the cup, noted the weight, and then added the Prussian blue into oil. This was much easier to mix in, but the viscosity seemed to vary. I began to suspect that the mixture was thixotropic when I tried to pour it into a container. Immediately after mixing it would drop off of my mixer, but when I would go to pour it in the container it would flow out about 2 drops, then quit, with the rest stuck to the cup wall. If left overnight a little bit more would flow down- but most was still stuck to the wall.
I next tried peanut oil, which is slightly more viscous than the corn oil. I started with an oil to blue ratio of 1:2.5, There was not quite enough blue, color so I went to a 1 oil: 3 blue mix. This mix was poured into a small plastic bottle. It did not pour quickly- but it did pour. The next day I went to dispense some, and it would not pour. I tapped the container against the side of the plate- accidently dropped it on the floor- and it then would pour. This confirmed the mix had thixotropic properties. I have been using this batch in scraping. It works as expected. It does not dry, even if left overnight. When I need to dispense some, I tap the container against the side of the plate. I have not yet had any problems with pigment clumping.
If you got this far congratulations. You have the persistence required for scraping.
I am not claiming that either corn oil or peanut oil are ideal, or that the ratio I tried is ideal. You have to start someplace.
Wanting to learn how to make Aqua wash more easily used, I started doing some homework and experimenting.
Paints and inks have been in use since the dawn of civilization. To make oil paint pigments are mixed with an “drying oil”. After application a drying oil reacts with oxygen causing it to thicken and then changes to a solid. Common drying oils include linseed oil, walnut, poppy, and safflower oil, in order of drying speed, fastest to slower. The drying time can be from days to months. Some drying oils are too thick to spread as a paint, so the paint is thinned with a solvent, which evaporates after application.
Charbonnel Aqua wash is a high-quality etching ink made with extra finely ground pigments. It is an emulsion of pigment and a “stand oil” modified to be washable with water. It does not contain water. The stand oil for most of the colors has a viscosity of 30 poise which is considered a medium viscosity. Some blacks also contain 200 poise stand oil, making them more viscous. Pigments can also change the viscosity of the ink. Prussian blue is harder to spread because of the pigment used.
The most important thing I learned from my experimenting is that Aqua wash is not drying during use. It thickens due to change in viscosity after it is applied because it is thixotropic. Thixotropic substances have a time dependent decrease in viscosity when shaken, stressed or shear stressed. The process of rolling out or smearing the ink on to a surface plate decreases the viscosity of the ink film for a short time. In other words, more energy put into spreading the ink leads to a greater decrease in viscosity. We typically will spot an object relatively soon after first applying the ink to the plate and from this get a sense of how thick the ink is. As we scrape, the ink returns to its normal “thicker” viscosity. When we later go back to spot the work again the ink has already been spread so we just work it to even the layer. This is not enough energy to decrease the viscosity, we notice the difference, and assume that it is from drying. To thin the blue on the plate we add our substance of choice (lets pick Windex) - and then mix it in, changing the viscosity by both dilution and initiating thixotropic changes. As we are scraping the Windex is evaporating out of the film on the plate, and the viscosity increases again as the blue is less diluted.
Given the above, our difficulties in getting consistent results are to be expected. This does not mean that Aqua wash is too difficult be used- it just means that we are still learning how to best use it. Richards preferred soft foam roller is going to have a difficult time with undiluted Aqua wash as it is unable to transfer enough energy to decrease the viscosity. Peter holding the brayer at an angle will increase the sheer stress and thus be more efficient at decreasing the viscosity. A harder brayer should also lower the viscosity more than a softer one.
Thixotropic changes are time dependent, meaning that when the shear stress begins, the viscosity decrease takes a consistent time to occur and reach equilibrium. When the shear stress ends the viscosity returns to its baseline state over a consistent time. We need to be aware that the amount of time since we spread the blue may determine its viscosity.
From my limited reading for thixotropic changes to occur requires the substances involved need to be within a certain range of proportions and physical conditions. The same substances in different proportions will not have the same properties and the same substances in the same proportions but different physical conditions will not behave the same. The only sure way to know what a change will do is to try it and see.
In adjusting the viscosity of Canode I usually use surface plate cleaner or lighter fluid, as I have both handy. I have found that the surface plate cleaner, water, and Windex will work for a while, and then you need to add more. I believe this is due to evaporation. The same is true for the lighter fluid and paint thinner- but the need to reapply occurs even faster, as they evaporate faster. When I started using Aqua wash I used the same substances to thin it- and found the same pattern with evaporation- but more variability in outcome.
Both Canode and Aqua wash are emulsions that can be thinned with either water or oil. Using an oil to change the viscosity should work better than using water-based solutions (Windex, surface plate cleaner) or solvents (lighter fluid) as oils evaporate very slowly. This means that whatever effect we get will be stable over time. There is no risk of rust with an oil.
Charbonnel sells the oil they use in Aqua wash separately to use in decreasing its viscosity. At $11 for 75 ml, it costs less than the ink. I have not tried it. It is a “stand oil” which is a type of drying oil.
For scraping, we do not need an oil that dries- even if it does take a week or two to dry to the touch. We need an oil that is less viscous than Aqua wash- and one readily available.
I have started experimenting with corn oil, and peanut oil. To my knowledge neither are considered a drying oil.
The Aqua wash is so thick and sticky that measuring it out by volume is not practical. I elected to measure by weight, using an electronic scale sold for measuring gunpowder. Hint: squeeze the tube to get some blue out, then “cut” it off with a piece of thin wire. The other problem with the stickiness is that it sticks to the side of the container, making it very hard to mix. I eventually heated some piano wire and formed it into a helix at the end. This was put in the chuck of a foredom tool. The helix can be run against the sides and bottom of the container. This worked well.
I first tried corn oil. In the first attempt I put the blue in the cup and gradually added oil until I had a 2:3 corn to blue ratio It was too thick to pour out of the container. Next I put the corn oil in the cup, noted the weight, and then added the Prussian blue into oil. This was much easier to mix in, but the viscosity seemed to vary. I began to suspect that the mixture was thixotropic when I tried to pour it into a container. Immediately after mixing it would drop off of my mixer, but when I would go to pour it in the container it would flow out about 2 drops, then quit, with the rest stuck to the cup wall. If left overnight a little bit more would flow down- but most was still stuck to the wall.
I next tried peanut oil, which is slightly more viscous than the corn oil. I started with an oil to blue ratio of 1:2.5, There was not quite enough blue, color so I went to a 1 oil: 3 blue mix. This mix was poured into a small plastic bottle. It did not pour quickly- but it did pour. The next day I went to dispense some, and it would not pour. I tapped the container against the side of the plate- accidently dropped it on the floor- and it then would pour. This confirmed the mix had thixotropic properties. I have been using this batch in scraping. It works as expected. It does not dry, even if left overnight. When I need to dispense some, I tap the container against the side of the plate. I have not yet had any problems with pigment clumping.
If you got this far congratulations. You have the persistence required for scraping.
I am not claiming that either corn oil or peanut oil are ideal, or that the ratio I tried is ideal. You have to start someplace.
