There are a number of problems with your workflow.
A scanned image is always raster, that is just the nature of
scanning. So, unless you are converting the scanned image to curves
in Illustrator there is no point in going through that program at
Since the scan is raster it’d be better to open it in Photoshop,
invert the colors, then bring it into Illustrator for conversion to
vector. However, since your are reducing the image quite a bit you
don’t need to convert to vector at all.
Etching can be thought of as a raster rather than a vector process,
so there is no great utility in converting a drawn image. Photographs
are always raster, and photo-etching starts with making a photo
negative, after all.
Do you produce your own film, or send the art out for photography?
Are you producing your own positives to give to an etching service,
or are you sending them the digital file?
Anyway, here’s my suggested workflow:
First, decide on the final size of the etched area. Then find out
what the optimal DPI (dots per inch) is for the imagesetter, or
printer, you’ll be using. Now you can calculate the absolute image
size you’ll be using. F’rinstance, let’s say the final etched area is
a 1"x1" square, and the imagesetter can output at 1200 dpi. That
means that the raster image which you’ll burn will have a size of
1200x1200 pixels. Each pixel will be translated by the imagesetter
into one dot in the 1200dpi matrix. Now you know how big your image
has to be in Photoshop.
Now you will set your scanner to scan at a DPI that is appropriate
for the size of your original. If you scan at 300 DPI then your
original needs to be 4"x4" to come to 1"x1" at 1200 DPI. If you scan
at 100 DPI then your original would need to by 12"x12" to come to 1x1
@ 1200dpi. Since you say you’re reducing about 1/3, then you’d set
your scanner to scan at 400 DPI to get to a 1200 DPI at print size.
Now, draw your pattern at the appropriate size, 3"x3" in our example
(3"x3" @ 400 DPI becomes 1"x1" when output at 1200 DPI). Scan the
drawing at 400 DPI and bring it into Photoshop. Clean up the image,
if necessary, and invert the image.
Here’s the critical difference in workflow, DO NOT scale or resize
the image. Image editors like Photoshop allow you to change the DPI
setting of an image without otherwise altering the image. Do a SAVE
AS… and only change the DPI (Print Size) setting of the image. This
way, no pixels will be changed, but the image will output at the new
resolution. So an image which was 3"x3" and scanned at 400 DPI, will
print at 1"x1" when output at 1200 DPI. And none of the pixels will
This only speaks to your digital workflow. If the problem is in your
photoresist or elswhere in the actual etching process, that’s a
different issue entirely.