How To Test Gold With Acid: The Definitive Professional Guide

Testing gold with acid involves scraping the metal against a novaculite touchstone to deposit a streak, then applying nitric and hydrochloric acid solutions of varying strengths to observe the chemical reaction. Authentic gold resists specific acid concentrations corresponding to its karat purity (10k, 14k, 18k, or 22k), while base metals, gold-plated alloys, or lower-karat counterfeits will dissolve, discolor, or turn green.

Essential Equipment and Safety Standards for Acid Testing

Acid testing relies on the chemical resistance of noble metals. Gold is highly resistant to oxidation and acid dissolution, whereas base metals like copper, zinc, nickel, and silver react rapidly when exposed to strong acids. To perform this test safely and accurately without damaging the jewelry or asset, you must gather the correct equipment and establish a controlled testing environment.

Mandatory Tools and Safety Gear

• Nitric Acid Testing Solutions: A standard gold testing kit contains separate bottles of acid calibrated for specific karats: 10k (approx. 10-15% nitric acid), 14k (approx. 30-35% nitric acid), 18k (approx. 50% nitric acid), and 22k (a modified formulation containing both nitric and a small fraction of hydrochloric acid to create a weak aqua regia).
• Novaculite Touchstone: A fine-grained, slightly abrasive, black siliceous stone used to safely shave off a microscopic layer of the metal for testing.
• Nitrile Gloves: Acid-resistant gloves (do not use thin latex, which can degrade rapidly upon contact with concentrated nitric acid).
• ANSI Z87.1 Approved Safety Goggles: Crucial for protecting eyes from accidental splashes or pressurized fumes when opening aged acid bottles.
• Sodium Bicarbonate (Baking Soda) Neutralizing Agent: A saturated solution of baking soda dissolved in water, or dry baking soda powder, kept within arm's reach to immediately neutralize any acid spills.
• Distilled Water: Used for rinsing the touchstone between tests.
• Jeweler's File: A fine-cut needle file to bypass outer plating layers on questionable pieces.
• Gold Comparison Needles: Solid metal keys tipped with verified karats (10k, 14k, 18k) used to run parallel comparison tests side-by-side with the unknown sample.

Environmental Requirements

• Ventilation: Conduct all testing under a fume hood or in a highly ventilated room with an active exhaust fan. Nitric acid reactions with copper or base metals release nitrogen dioxide ($NO_2$), a toxic, reddish-brown gas that causes respiratory irritation.
• Acreage and Budget: A professional starter setup costs between $30 and $60. The testing process takes roughly 5 to 10 minutes per item.

Step-by-Step Gold Acid Testing Protocol

To avoid devaluing fine jewelry or numismatic coins, never apply acid directly to the surface of the item unless you have already filed deep into an inconspicuous area of a suspected plated counterfeit. The touchstone method described below is the industry standard for non-destructive surface testing.

Step 1: Prepare the Testing Workspace and Safety Gear

Put on your safety goggles and nitrile gloves. Ensure your workspace is a flat, chemical-resistant surface (such as glass, ceramic, or stainless steel) free of clutter. Arrange your acid bottles in order of increasing karat strength (10k, 14k, 18k, 22k). Prepare a small plastic container filled with water and two tablespoons of baking soda to serve as your acid neutralizing bath.

Step 2: Perform the Scratch Test on the Touchstone

Select an inconspicuous area of the gold item, such as the inside of a ring band, the jump ring of a pendant, or the clasp of a chain. Hold the jewelry firmly and rub it back and forth against the black touchstone with moderate, even pressure.

• Create a solid, thick metal streak that is roughly 1 inch (2.5 cm) long and 0.1 inches (2.5 mm) wide.
• If you have gold comparison needles, select the needle that matches the stamped karat on the jewelry. Scratch a parallel control line of equal thickness directly next to the sample streak, leaving a 0.25-inch gap between them.

Warning: Gold plating can be exceptionally thick (heavy gold electroplate or gold-filled). A light scratch will only test the outer layer of pure gold. If you suspect an item is plated, use a jeweler's file to make a small, deep notch in an obscure area of the piece and scratch the stone using the exposed inner metal of the notch.

Step 3: Apply the Acid Solutions Systematically

Uncap the acid bottle corresponding to the stamp on the jewelry. If the item is unstamped, start with the 10k acid bottle to avoid destroying the streak immediately if it is a low-karat alloy.

• Hold the acid bottle directly over the scratch on the touchstone.
• Gently squeeze the bottle to deposit one small drop of acid across the gold streak.
• Ensure the plastic dropper tip of the bottle never physically touches the metal streak or the stone. This prevents copper or nickel contamination from entering your acid bottle, which will spoil future tests.

Pro-Tip: If the streak on the stone is long enough, you can apply different acids to different sections of the same streak. Apply 10k acid to the top third, 14k to the middle, and 18k to the bottom to determine the exact karat of an unmarked piece in a single run.

Step 4: Interpret the Chemical Reactions

Observe the reaction on the touchstone immediately, and continue to watch it for 30 seconds. The reaction of the acid with the metal streak will reveal its composition:

• No Reaction: If the metal streak remains completely bright, visible, and unchanged under the acid, the gold is equal to or higher than the karat rating of the acid bottle used.
• Slow Discoloration or Fading: If the streak slowly turns brown, grey, or partially fades over 10 to 20 seconds, the gold is of a lower karat than the acid bottle used (e.g., a 12k gold alloy tested with 14k acid).
• Instant Dissolution or Green Bubbling: If the streak completely vanishes within 2 to 5 seconds, or if the acid turns bright green and bubbles, the streak is composed of brass, copper, nickel, or a gold plate containing very little precious metal.

Step 5: Clean and Neutralize the Testing Surfaces

Once the test is complete, you must neutralize the active acid on the touchstone before wiping it clean.

• Submerge the touchstone directly into your prepared baking soda bath, or pour a small amount of baking soda powder directly onto the stone. You will see a brief effervescence (fizzing) as the acid is neutralized.
• Rinse the stone thoroughly with distilled water.
• Wipe the stone completely dry with a clean paper towel.
• To restore the deep black luster of your touchstone and prepare it for future tests, rub a single drop of mineral oil or light baby oil into the stone's surface, then wipe off any excess with a clean cloth.

Gold Acid Reactivity and Karat Threshold Matrix

The table below outlines how various gold alloys and base metals react to standard testing acids. Use this matrix to analyze your scratch results.

Acid Testing Pitfalls and Field Corrections

Operating an acid testing kit requires consistent calibration and an awareness of false positives. Below are real-world failures encountered during testing and how to resolve them.

• Scenario 1: A gold-plated brass item passes the touchstone test as solid gold.

  
  
  
  • Root Cause: The scratch applied to the touchstone was too light, scraping off only the thick outer layer of gold plating (such as 1/20 12k gold-filled) without reaching the brass core.
  • Actionable Fix: Perform a "destructive" file test. Use a fine-cut jeweler's file to make a deep notch in an inconspicuous area of the piece. Place a drop of 14k or 18k acid directly into the notch. If the core of the metal bubbles green or turns black, the item is plated.

• Scenario 2: The acid yields a sluggish, ambiguous reaction, or fails to dissolve known base metals.

  
  
  
  • Root Cause: The testing acid has expired or degraded due to exposure to heat, sunlight, or air contamination. Nitric acid has a shelf life of approximately 1 to 2 years once opened.
  • Actionable Fix: Verify your acid's potency using a solid gold testing needle of a known karat or a verified gold coin. If the 14k acid fails to react with a known 10k gold control streak, discard the bottle safely at a local hazardous waste facility and replace your kit.

• Scenario 3: The touchstone is permanently stained or retains faint ghost streaks from previous tests.

  
  
  
  • Root Cause: Acid was left on the stone too long, or the stone was not neutralized and washed properly, allowing metal oxides to bind to the porous surface of the novaculite.
  • Actionable Fix: Wet the touchstone with water and sprinkle a small amount of baking soda or scouring powder on it. Using ultra-fine 320-grit wet-and-dry sandpaper, sand the surface of the stone flat under running water. Rinse, dry, and apply a light coat of mineral oil to restore the surface.

Frequently Asked Questions

Does acid testing damage real gold?

If performed correctly using the touchstone method, acid testing does not damage the jewelry itself. Only a microscopic amount of metal is scraped off onto the stone. However, if you apply acid directly to the jewelry item, it will leave a permanent dark brown or black blemish on any alloy below 18 karats.

What acid is used to test gold?

Nitric acid ($HNO_3$) is the primary chemical used to test gold. For testing higher karats like 18k and 22k, a mixture of nitric acid and a tiny fraction of hydrochloric acid ($HCl$) is used, mimicking a highly diluted form of aqua regia to break down the highly stable gold alloys.

How do you tell the difference between 14k and 18k gold with acid?

Scratch the unknown item onto the touchstone. Apply a drop of 14k acid to the streak; if it does not react, the item is at least 14k gold. Apply a drop of 18k acid to a different part of the same streak; if the streak slowly turns brown or fades away, the item is 14k gold. If it remains bright and unaffected by the 18k acid, the item is 18k gold or higher.

Can you test gold with household vinegar?

Vinegar, which contains weak acetic acid, is not strong enough to reliably test gold. While it may slowly discolor base metals over several hours, it cannot differentiate between different gold karats, nor can it detect professional gold-plated copper, brass, or lead alloys.

Why did my gold scratch turn brown on the stone?

A brown reaction indicates that the gold alloy is of a lower purity than the rating of the acid bottle you used. For example, if you apply 14k acid to a 10k gold scratch, the higher concentration of nitric acid will dissolve the copper and silver alloys within the 10k streak, leaving behind a degraded, brown-colored residue of fine gold.

Elevate Your Precious Metal Verification Accuracy

To secure your margins and guarantee absolute accuracy when buying, selling, or appraising estate jewelry, pair your manual acid testing kits with high-precision digital gold testers. Investing in professional-grade testing suites ensures you can confidently authenticate any precious metal asset that crosses your desk.