Is it true that "Red glazes are unsafe for tableware"? Explaining the difference between Overglaze and Glaze

Conclusion: Red Glazes are Safe

We often get asked, "Is it safe to use red glazes on pottery intended for tableware?"

The answer is "YES". You can use them with confidence.

This misunderstanding stems from confusing modern "Glazes (Yuyaku)" with the harmful substances used in old-fashioned "Overglaze (Uwae)" techniques.

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Why is red thought to be dangerous?

Old overglazes contained harmful substances

There are three main ways to color ceramics:

1. Underglaze (Sometsuke) / Painting under the glaze

• Paint with Gosu (Cobalt) on bisque ware, then apply a transparent glaze over it.
• Fired at high temperatures (over 1200°C).
• The paint is sealed under the glaze layer, protected by glass.
• Arita ware's Sometsuke (blue painting) is a typical example.
• → Highly Safe

2. Glazing

• Cover the entire vessel with a colored glass-like layer.
• Fired at high temperatures (over 1200°C).

3. Overglaze (On-glaze painting)

• Painting on the surface of an already fired vessel.
• Fired at low temperatures (around 800°C).

The problem was with the "Overglaze".

Old overglaze paints contained substances such as:

• Lead: Used to add gloss.
• Cadmium: Used to produce vivid red colors.
• Selenium: Also used for red colors.

These could sometimes leach out slightly when in prolonged contact with acidic foods like vinegar or fruit.

That is why the image of "Red = Dangerous" spread.

How about now?

Since the law changed in 2009, pottery made in Japan cannot be sold unless it passes strict inspections.

• Overglaze paints have also changed to use non-hazardous substances.
• Inspection method: Confirming that no harmful substances leach out even after soaking in vinegar for 24 hours.

In short, modern Japanese pottery is safe, whether it is overglaze or glaze.

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There are actually 3 ways to make red glaze

What is the difference between Glaze and Overglaze?

It's not just "Glaze is safe" and "Overglaze is dangerous".

What matters is the materials used and how they are made.

Actually, there are 3 main ways to produce red color in glazes. And each has a completely different coloring mechanism.

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1. Red made with Iron (Amber to Reddish Brown)

The most familiar red color

What kind of color?

• Amber, brownish brown, reddish brown
• Astringent red like persimmon
• Warm brownish red

What is used?

• Iron (Iron Oxide)
• The same substance as iron rust.
• Also called Bengara (Red Ochre).

How does the color appear?

• The metal itself colors through a chemical reaction.
• Iron is mixed into the glaze and fired at a high temperature of 1200°C or higher.
• The iron undergoes a chemical reaction within the glaze.
• This reaction creates a reddish-brown color.

Familiar example

• The color of rusted iron
• The color of red brick
• Brownish red

Why is it safe?

• Iron itself is a safe material.
  
• It is sealed inside glass at high temperatures over 1200°C.
• Since iron integrates with the glass, there is no worry of leaching.

Typical glazes

• Ame-yu (Amber glaze): Amber colored glaze
• Kaki-yu (Persimmon glaze): Persimmon-like reddish brown
• Tetsu-aka-yu (Iron red glaze): Reddish brown with strong red tones

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2. Red made with Copper (Shinsha / Copper Red)

Beautiful red requiring advanced technique

*For notes on firing glazes containing copper, please see this article (Copper Volatility and Measures) for details.

What kind of color?

• Vivid red like blood
• Deep reddish purple
• Very beautiful and luxurious

What is used?

• Copper (The same material as a 10-yen coin)
• Mixed into the glaze at about 1-3%.

How does the color appear?

• The metal itself colors through a chemical reaction.
• A special method called "Reduction Firing" is used.
• The oxygen in the kiln is extremely reduced.
• Then, the copper chemically reacts and returns to its original copper red color (the color of a new penny).
• This becomes a beautiful red color inside the glaze.

Familiar example

【Easy to understand with a penny (10-yen coin)】

New penny → Shiny copper red (This is the reduction state)
Old penny → Discolored to green (This is the oxidation state)

Same with glaze:
Reduction firing → Copper turns red
Oxidation firing → Copper turns green

Why is it safe?

• Copper itself is a safe material (in small amounts).
  
• It is sealed inside glass at high temperatures over 1200°C.
• Since copper integrates with the glass, there is no worry of leaching.

Typical glazes

• Shinsha-yu (Cinnabar glaze): Blood-like red
• Traditional technique handed down from the Song Dynasty in China
• Because it is a difficult technique, it is often found in high-end products.

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3. Red made with Pigment (Our Glaze)

Vivid and safe red made with the latest technology

What kind of color?

• Very vivid red
• From orange-red to deep crimson
• As vivid as the old cadmium types

What is used?

• Encapsulated Pigment (Inclusion Pigment)
• This is a method using "Color source / Pigment".
• Pigment where the color source is sealed inside a special ceramic capsule.

Difference from Iron and Copper

	Iron / Copper	Encapsulated Pigment
Material	Metal itself	Color source (Pigment)
Coloring Mechanism	Metal reacts chemically	Uses the pigment's color as is
Variety of Colors	Limited colors	Many colors can be made
Firing Method	Copper requires reduction firing	OK with both Oxidation and Reduction

Why is it safe?

This is the same mechanism as a medicine capsule.

To explain simply:

【Same as M&M's Chocolate】

M&M's:
Chocolate → Wrapped in a sugar shell → Hands don't get dirty

Encapsulated Pigment:
Color source → Wrapped in a special ceramic shell → Does not leach out

Furthermore!

This "pigment in a capsule" is fired at ultra-high temperatures of over 1200°C and solidifies like glass.

In other words, it is doubly protected:

1. 1st Wall: The color source is inside a special ceramic capsule.
2. 2nd Wall: The capsule itself is trapped inside the glass-like glaze.

With this, it won't leach out even with vinegar or fruit juice.

Merits

• Can produce vivid colors
• Wide variety of colors
• Lead-free
• Selectable firing method (OK with both Oxidation and Reduction)

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Where is the difference between Glaze and Overglaze?

Easy comparison table

	Red Glaze	Overglaze (Old type)
Firing Temp	Very high (Over 1200°C)	Relatively low (Around 800°C)
Coloring Mechanism	1. Iron/Copper: Chemical reaction 2. Pigment: Encapsulated	Painting pigment on the surface
State of Color	Integrated with glass, or Capsule further inside glass	Just sitting on the surface
Lead	Not used	Used in the past
Leaching	Almost no leaching	Possibility with old ones
Analogy	Soda inside a candy	Sticker on the surface of a candy

Summary of why red glazes are safe

1. Iron Red

• Iron itself is a safe material
• Metal colors through chemical reaction
• Integrates with glass over 1200°C
• A mineral necessary for the human body

2. Copper Red

• Copper itself is a safe material (small amount)
• Metal colors through chemical reaction
• Integrates with glass over 1200°C
• Becomes beautiful red with reduction firing

3. Encapsulated Pigment Red (Ours)

• Color source is sealed in a ceramic capsule
• Further vitrified over 1200°C
• Double containment structure

Common point: All vitrified at high temperature

• All reds are fired at high temperatures over 1200°C
• Solidifies like glass
• Materials do not come out

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FAQ

Q1. Can I tell the difference just by looking at a red vessel?

A. It is easy to distinguish.

Glazed Vessel:

• Surface is smooth, like glass
• Color is uniform throughout the vessel
• Color does not come off when rubbed

Overglazed Vessel:

• Pictures or patterns are drawn on top of the glaze
• You can sometimes feel a slight unevenness when touching the painted part
• Modern ones use safe paints

Q2. Which type is Fukaumi Shouten's glaze?

A. We use the latest type using Encapsulated Pigments.

Our red glazes use Encapsulated Pigments (Inclusion Pigments).

Compared to Iron or Copper reds:

• Can produce more vivid red colors
• Wider range of colors (from orange-ish to deep crimson)
• Even safer with double containment structure
• Firing method doesn't matter

Q3. Which is safer, Iron/Copper or Encapsulated Pigment?

A. Both are safe.

For Iron/Copper:

• Metal itself is safe
• Integrates with glass through chemical reaction
  

For Encapsulated Pigment:

• Color source is doubly contained
• Ceramic capsule + Vitrification
• Structure where it physically cannot get out

Both are fired at high temperatures over 1200°C and vitrified, so the safety level is the same.

Q4. Are colors other than red safe?

A. Yes, they are safe.

Actually, it was mainly "Red" that had problems in the past.

Reason: It was difficult to produce vivid red, so harmful materials had to be used.

But now, all colors are made with safe materials and technology.

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Summary: Please use with confidence

There are 3 types of red glaze, and mechanisms differ

1. Iron Red (Metal chemical reaction)

• Amber, brownish brown, reddish brown
• Iron colors through chemical reaction
• Warm astringent red

2. Copper Red (Metal chemical reaction)

• Blood-like vivid red
• Copper reacts chemically in reduction firing
• Requires advanced technology

3. Encapsulated Pigment Red (Sealed color source)

• Very vivid red
• Double containment structure
• Latest safety technology

Safety common to all

✓ Fired at high temperatures over 1200°C
✓ Solidifies like glass
✓ Materials do not come out
✓ Clears national standards
✓ Lead is not used

"Red = Dangerous" is a story of the past

The problem was with Lead, Cadmium, and Selenium used in Overglazes.

Red glazes are safe because the coloring mechanism and structure are completely different.