When your emulsion is acting up you need help fast. If you don’t have time to read all of this, assume your emulsion is under-exposed. That’s right! I went there. If you do have time to read this, I am going to discuss some common and not so common emulsion problems. I may even add photos as I am really working hard these days. Well, that and I realized that cell phones take pretty decent pics and they can often tell a story. Who knew?
Before jumping into the problems, you know I have to ask…how are you testing exposure time? Please tell me you aren’t rubbing white paper towels over top of it looking for color transfer. That is soooooooo old school. How about we get you a 21 Step Sensitivity Guide? It’s not high-tech. It’s not sexy. It is pretty darn effective though. Not so good at math? It’s ok, my cell phone also has a calculator. Technology, right? E-mail me a photo of your rinsed out 21 Step Sensitivity Guide and how long you exposed the emulsion for. I will work it out and even show you how I did the math. My seventh grade algebra teacher will be so thrilled as I never did this for her class. Do one thing for me though, rinse the test image area pretty hard. Don’t hold back. This will give you an accurate result to work with.
Pictured above is my famous 21 Step Sensitivity Guide. Each rectangle is a gradually darker/denser tone of black. The darker the rectangle, the easier it will be to rinse out after you expose the image. The goal is to rinse out this image and be left with #2 through #7 still in the screen. #8 and below should rinse out. If that happens and you rinsed hard, you will be free of over and under-exposure problems. If not, the back of the envelope provides instructions to increase or decrease exposure time depending on your result.
Alright, on with it already! Here are the most common emulsion problems and causes:
Little dots of missing emulsion. This is a frustrating little problem as you may ruin numerous prints before noticing the problem. Luckily, this is an easy problem to stop in its tracks. First, clean your exposure glass. Sprayway Glass Cleaner is pretty awesome stuff if you want the best. Regardless, any speck of anything on that glass will expose into the emulsion. Second, check your film positives for any stray dots. This can happen with both inkjet and laser printers. If those two options don’t change anything, you have a screen prep problem. Degrease the screen and be sure to rinse with plenty of water. Keep in mind a pressure washer does not use as much water as it appears. A regular hose would be great to flood the screen. If you are not degreasing your screen, you simply will always be riddled with pinholes among other problems.
One alternative answer could be contamination in your emulsion bucket. Perhaps you were coating a screen which was not clean. The contamination from this screen made it into your scoop coater. You poured the excess emulsion back into the bucket. Now you have a bucket of emulsion with built-in pinholes. Not good.
Yet another possibility is old emulsion. I would only consider this if there are pinholes almost everywhere. A diazo or dual-cure emulsion has about 8 weeks of shelf life once it is mixed. When it goes bad, you may see a screen riddled with pinholes or it may simply not expose.
Residue in Image Area
If you are fortunate, this is just a sign that you need to rinse the image area out better. If you are less fortunate, this is due to under-exposed screens. If that is the case, reclaim this screen immediately. Otherwise, the moment you decide to clean this out with an ink remover or screen opener it will lock into your screen. You will not be able to get this back out. I would much prefer exposing a new screen over throwing a good screen in the trash. Be very cautious.
Many of my customers have called me in the past blaming the ink for not being able to clear a screen properly. This problem was the cause in most of these cases. My ink is pretty good, promise!
Ink Building Up on the Image Area Edges
Here is another case of what can under-exposed screens do for you! I’ll tell you what…it can give you crappy prints! When the emulsion is really soft around that image area, the ink wants to stick to it. As with the residue in the image area, be very cautious as solvents can lock this into your screen mesh. You don’t want that.
Another possibility is your film positive. Take a close look. Are the edges of the toner or ink fuzzy? Obviously this could be the cause if that is the case. Also, are you getting tight contact between the film positive and the emulsion? Check your vacuum blanket for holes and keep a close eye on your exposed emulsion. If you start seeing the edges of the film itself show up when rinsing, you may not have tight contact. You also may be under-exposed. A picture is below demonstrating this appearance. The line down the middle is exactly where the film positive was pressed against the emulsion.
So you got me on this one. This is usually a sign of over-exposure. NOT ALWAYS, but most of the time. Obviously, if you have a very coarse mesh, you will get some bit of sawtoothing every time. Picture a 38 count glitter screen. Sawtoothing…so much sawtoothing. Aside from these extreme cases, exposing too long is very likely. That 21 Step Sensitivity Guide is calling your name. It is a life-saver! A photo of a print is below demonstrating sawtoothing.
Image Area Difficult to Rinse
Here is an easy one to solve. Either the screen has been over-exposed or the film positive was not dark enough. The concept is simple, light exposes emulsion. If the image area emulsion gets exposed, it will be tough to impossible to rinse out. Clearly light is getting to it somehow if you are having this problem.
An overlooked idea is that light may be getting to the image area before you ever go to expose the screen. Check your dark room and make sure no light (except safe light) is reaching your coated screens.
Emulsion is Bubbling
Your screen is really, really under-exposed. Again with the 21 Step Sensitivity Guide! You really need to expose properly.
Other Ideas to Consider
- If your emulsion feels really slimy when you are rinsing out the image area, it is under-exposed.
- If you are struggling to reclaim a screen, it may be under-exposed. All solvents will harden emulsion when it is under-exposed. Also consider the cleaning products you use. If they are harsh solvents and your emulsion is not solvent resistant, this can create the same problem.
- You always expose for the same amount of time but only some screens are under-exposed. Consider your humidity. Were the screens ever really dry when you exposed them?
- If a screen is impossible to reclaim, did you allow the emulsion remover to dry? If so, it is locked-in for good.