Double coating with dichromate

Keep in mind, it's the total amount of dichromate that you put into the tissue that matters. If you split that same amount up into 1, 2, 3, or even 4 coatings, it won't really make a difference, unless you wait quite some time between coatings, and then you risk some chemical fog.

If you're asking what the effect of doubling the amount of dichromate has, it will reduce contrast by the equivalent of about 1/2 a grade or so, and will increase speed roughly 1/6 to 1/3 of a stop.

I don't want to discourage discussion on this forum, but you may want to check out the Bostick & Sullivan carbon printing forum, too. It's a great resource.

Good luck with your learning...It's been about a month since I took my first carbon printing workshop, and if I would quit "trying something new" and just settle down and get dialed in, I'd be printing real images instead of step wedges.

--Greg

 

I take the amount I brush onto the tissue and divide it to two parts, brush on half until the tissue is dry (no surface moisture) then brush on the second half. Sandy King recommends this method. I do not know his reason, but I do it so that none of the liquid runs off the tissue.

But give it a try and compare it to doing it all at once -- and let us know if there is any difference!

Vaughn

 

If you're soaking the tissue, the answer gets a little more complex, and would require a little more testing.

It's possible, depending on dichromate concentration, temperature, etc., that the gelatin is absorbing most (or all) that it can during the first bath. In that case, a second bath would only contribute a little, or not at all. The same result could be accomplished by soaking longer in the first bath.

So you won't be wasting dichromate, per se, but you might be wasting time and effort in the second bath and second drying time.

--Greg

 

I have always assumed ( a dangerous thing LOL! ) that the dichromate gets absorbed into the tissue fairly easily. If this is the case, double-dipping, so to speak, would only increase the amount of dichromate that gets into the tissue -- not necessarily any deeper nor more evenly distributed (but it would be interesting to do a controlled test to see if this is the case or not.) So double dipping may have no different effect than increasing the concentration of the dichromate for a single application, as Greg suggests.

Vaughn

edited after reading greg's post:

Actually, if the tissue is completely dried after the first soak, the second soak might absorb about the same amount of liquid as the first soaking -- so it might double up the dichromate load in the tissue. Thanks for the question, Michael!

 

I had always assumed there was a physical limit to how much dichromate could be absorbed, but Vaughn may be right--the water may be limiting that. Drying in-between multiple baths might increase that limit.

Mike, since Vaughn and I use a brush instead of a bath, please try it...maybe you could teach us something new!

--Greg

 

I mix my dichromate with water, but then dilute it with rubbing alcohol so the tissue dries faster. Seems I read a thread on the B&S carbon forum that the tissue absorbs more sensitizer if the solution is very cold. You can rummage around the B&S forum to find the thread. I'm sure it's there.

John

 

If you soak the tissue there is a finite amount of dichromate that can be absorbed. How much depends on the temperature of the solution, its strength, and whether or not it is diluted with alcohol or acetone.

When I started the carbon discussion group on Yahoo last August or September one of the first things I did was post some test data on this topic. I had previously posted some tests along the same line on the B&S site but my notes there are not as extensive as on the Yahoo site.

Although there are some advantages to soaking the tissue I continue to sensitize with the spirit method by brushing a dichromate solution plus acetone solution on the tissue. This method gives excellent results, though it requires some experience, as does coating for alternative processes like pt/pd, kallitype, vandyke, etc. However, with the spirit method you get the shortest drying time, and the method is more environmentally friendly than other methods because it uses an absolute minimum of dichromate and none is discarded.

And Vaughn, the reason I divide the coating into two sessions separated by about five minutes to allow some drying is to get more dichromate into the tissue without having to brush for a very long time with double the amount applied once.

Sandy King

 

Thanks Sandy. When I read about your method of dividing the sensitizer in half, it was like, "Of course! Why did I not think of that?!" Much easier to do it that way and not have sensitizer running all over the place! I don't wait the 5 minutes, though.

Vaughn

 

If you dilute the dichromate solution with alcohol 1:2 it will dry faster than diluted 1:1. It will also dry faster mixed 1:1 with acetone.

A lot of cool water (65F or less) is very useful when printing with carbon. In the summer here at my home in South Carolina the water temperatures are in the 75-77F range and this complicates printing quite a bit. RH in the house is also fairly how this time of year, which also complicates printing because the tissue takes longer to dry. Because of this I print more with the iron processes (kallitype, pt/pd and vankey) in the summer months. These processes love high RH and warm temperatures.

Sandy King

 

I also have followed Sandy's recommendation on coating with acetone and waiting between coats as he and Vaughn have said. Here is something I've wanted to ask for a while. Can one use two different dichromate strengths on the same image? By this I mean I have an image that has an area that needs a low strength and one that needs a higher strength to give more or less contrast to the final print. The areas are almost evenly split in the image and it will be easy to coat the tissue with this "split coating". I just was curious if anyone has tried this? I'll report my findings. I hope to start printing again tomorrow.

Jim

 

Jim -- don't see why one could not work that way. The differently sensitized parts of the tissue would have slightly different printing speeds -- one might need to burn in the section sensitized with a lower concentration.

When brush sensitizing with Acetone 1:2, the evaporation of the acetone cools the tissue down nicely if the room is warm.

 

Vaughn, my carbon brother thanks for confirming my ideas. Not a lot of motivation to print right now so I'll let you know how it turns out when I get there. Happy fathers day to you.

Jim