Kelp carbon process – A direct positive carbon photographic process

Kelp carbon is not for the beginner. The kelp carbon process is a direct positive carbon photographic process. Jim Patterson shares his process.

Writer and photography / Jim Patterson

In short: Sodium alginate (from kelp) is the Colloid and Ferric (iron iii) Oxalate is the Sensitizer. The image is pigment bound to a support in the hardened (insoluble) colloid.

What is Kelp Carbon?

Traditional carbon prints are negative working; a negative transparency is needed to produce the positive image. Pigmented gelatin is coated on a temporary support, sensitized with a dichromate (or DAS more recently), exposed to UV through a negative, soaked in warm water briefly, transferred to a receiver sheet, rested, then developed in warm water as the temporary support is peeled off. The image is pigment bound to the final support in hardened (insoluble) gelatin.

Kelp Carbon is a direct positive carbon type process. A positive transparency is used instead of a negative and no transfer is needed for the image. An iron salt is the Sensitizer and Sodium Alginate is the colloid to hold the pigment. Pigmented sodium alginate is coated on the final support, hardened (made insoluble) by the iron salt, exposed to UV through a positive, and developed in mildly alkaline water with a soft brush. The image is pigment bound to the support in hardened alginate.

Kelp carbon print by Jim Patterson
Kelp carbon print by Jim Patterson. “Aunt Sally’s Shack” , digital camera, digital positive transparency, HPR paper.

COLLOID: add 3 ml glycerin to a beaker and add distilled water to the 300 ml mark. Add 6 grams of Sodium Alginate by slowly adding a small stream of powder at the outer edge of the vortex of the rapidly stirred liquid in an electric blender or with a whisk. Adding large amounts at once will cause clumps that dissolve very slowly. Let stand overnight to fully hydrate. This is a hazy clear viscous solution.

PLASTICIZER: Glycerin (glycerol) – necessary to prevent cracking of the coating when flexed.

SURFACTANT: Polysorbate 20 (Tween 20), helps the coating spread without holes.

SAFELIGHT: yellow bulb or 40-watt tungsten; avoid daylight and fluorescent light

STOCK SENSITIZER: Make a 10% stock solution by dissolving 20 grams of Ferric Oxalate in 200 ml distilled water (dissolves slowly over hours; an electric stirrer helps). Store in a brown glass bottle.

WORKING SENSITIZER: (1% FO) add 20 ml Stock Sensitizer to a beaker. Add distilled water to the 200 ml mark and mix well.

DEVELOPER: dissolve 1/2 teaspoonful of Sodium Bicarbonate (baking soda) in 500 ml distilled water. Dilute further if needed to make the developer less aggressive.

PAPER: hot press watercolor, Hahnemuehle Platinum Rag, Revere Platinum, Arches Platine, etc. Buffered papers interfere by precipitating iron salts and need acidifying to work.

PAPER SIZING: suspend 3 grams of arrowroot starch in 100 ml of cool distilled water and heat while stirring until it gelatinizes and becomes translucent. Avoid boiling. Cool to room temperature. Mix 1 volume of starch with 1 volume of Gamblin PVA Size. Then dilute this mixture with an equal volume of distilled water. Example: 25 ml starch + 25 ml PVA + 50 ml water. Add a drop of Tween 20. Stir well before using. Coat paper with a hake brush and hang to dry by a corner. After it’s dry, repeat the coating and hang by the opposite corner to dry.

NOTE: dried alginate does not adhere to many surfaces and is used as a mold release for casting and for dental impressions (doesn’t stick to your teeth). The pigmented coating did not stick to paper sized with polyvinyl alcohol/borax, gelatin/glutaraldehyde, fixed photo paper, Yupo, Pictorico White, Adox Art Baryta, casein, or k-carrageenan. I didn’t try adding starch to the aforementioned sizings. Plain arrowroot works, but is very matte and stains easily. 2% Agar with 1% starch works. Zoe Zimmerman’s matte albumen also works, leave out the citric acid and salt, and harden with alcohol after coating.

PIGMENT: tube watercolor, dry pigment, or India ink

PIGMENT COATING: Add 3 drops (0.15 ml) of India ink and 1 drop of Tween 20 to a shot glass. Add 10 ml Colloid and mix well with a stirring rod. Let stand a few minutes for the bubbles to resolve. This is enough for a 6×8-inch print.

  1. Coat the sized paper using a #100 wire wound coating rod. Dry taped or pinned down flat to prevent curling as it dries.
  2. Soak the dry-coated paper in a flat-bottomed tray for 30 minutes in the Working Sensitizer in the dark or dim room. Several papers can be sensitized at once by interleaving a sheet of cloth-like paper towel between each sheet, avoiding air bubbles. Wash in a tray of distilled water for 1 minute to remove excess FO after soaking. Tape or pin down to prevent curling as it dries in the dark. The coating is now insoluble in water and is UV sensitive.
  3. Expose through a Positive transparency to UV light in a print frame. My exposure time is about 45 minutes with a homemade BL tube unit.
  4. When placed into a tray of developer the exposed areas begin to hydrate and form a gel. Let it soak for 3 minutes and then use a soft hake brush wet with developer to remove the gel. Even though dry alginate doesn’t adhere to most surfaces, the gel sticks. So mild abrasion is needed to remove the exposed areas. The image can be manipulated with a small stiff brush. When developed wash in a tray of 2.5% vinegar (dilute 1 part white distilled Vinegar with 1 part water) for 5 minutes to harden the image and remove residual chemicals, followed by a 2-minute tap water wash. Let dry and marvel.

Ferric Oxalate is toxic if ingested. Most other materials are edible and vegan.

HOW: brown algae (kelp, etc) has insoluble alginic acid in the cell wall. Treatment with sodium hydroxide produces a water-soluble gum: sodium alginate. Calcium, strontium, barium and ferric ions respectively produce increasingly insoluble alginates. Ferrous ions form a weak crosslink that is more soluble than ferric ions.

When the sensitized pigmented coating dries, it is insoluble ferric (iron iii) alginate which traps the pigment. The UV-exposed areas become ferrous (iron II) alginate and/or ferrous oxalate (insoluble, and thus not a crosslinker).
Exposed areas hydrate and form a gel that is abraded away.


  • Sodium Alginate powder (food grade)
  • Glycerin
  • Polysorbate 20 (Tween 20)
  • Ferric Oxalate (Bostick & Sullivan) TOXIC IF INGESTED
  • Arrowroot starch
  • White distilled vinegar
  • Gamblin PVA Size
  • Distilled water
  • Sodium Bicarbonate (baking soda)
  • Pigment
  • Smooth paper, without buffers
  • A dim room
  • A UV exposure unit or sunshine
  • Print frame
  • A blender or whisk
  • A coating rod or uniform thickness coater
Kelp carbon print by Jim Patterson
Kelp carbon print by Jim Patterson. “The Old Home Place”, digital camera, digital positive transparency, HPR paper

NOTE: hard tap water has calcium ions that crosslink the alginate, thus the use of distilled water in the solutions. If you have soft water you may not need the distilled water.

With a positive process, it is important to have a smooth, uniform thickness of the coating, such as with a wire wound coating rod unless you want brush marks.

Jim Patterson is from New Orleans, is a semi-retired physician working work 3 days a week in an urgent care. Jim has a lifelong love of analog, alt photo, and the chemistry of it. See his website here.

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