Based on the highly specific information you provided, this sounds like a Northern Song Guan Kiln Long-Necked Bottle with Spiral Patterns. Its most striking feature is the powerful contrast in volume between its sturdy lower body and its slender neck.

Combining your precise measurements, we can clearly outline its unique form:

Feature DimensionSpecific DescriptionProportion & Visual AnalysisOverall ShapeLong-necked bottle with spiral patterns, total height: 21 cm.The height aligns with classic dimensions for Song dynasty display porcelain.Lower BodySturdy and robust, diameter approx. 12 cm, height approx. 10 cm.This is the visual "center of gravity," appearing full, stable, and powerful.Upper NeckSlender and long, diameter only approx. 3 cm.Creates a strong visual contrast with the body, appearing erect, elegant, and full of tension.Foot RingDark-colored (typical of "iron foot" characteristic).Exposes the dark brown or blackish clay body, an important identifier for Guan ware.Core DecorationSpiral patterns (also called "bow-string" patterns).Typically adorn the neck or shoulder, adding fine layers and a sense of rhythm.

Analysis of Form, Craftsmanship, and Meaning

This exaggerated "slender top, sturdy bottom" form is not arbitrary but a masterpiece of Northern Song Guan ware, blending archaic style with contemporary spirit:

• Archaism and Innovation: The shape may trace its origins to ancient bronze or jade vessels, but the extreme proportional contrast showcases the bold artistic refinement and abstract capability of Song dynasty artisans.

• Technical Challenge: In ceramic craftsmanship, throwing the heavy body and the thin, long neck as a single piece on the wheel, and maintaining stability without cracking during high-temperature firing, was extremely difficult, resulting in a very low success rate.

• Aesthetic Concept: The overall posture resembles a cultured scholar, conveying the idea of "rounded exterior, upright interior;沉稳 (steady) and挺拔 (tall and straight)." The round lower part symbolizes inclusiveness and depth, while the slender neck symbolizes integrity and moral character, perfectly embodying the ideal personality of Song dynasty literati.

Key Points for Examination and Appreciation

For such an object, you can begin your examination by focusing on the following key details:

1. Glaze Texture and Color: Typical Northern Song Guan ware has a thick, opaque, creamy glaze with a jade-like quality. The glaze color should primarily be in the celadon range, such as sky blue or powder blue, serene and profound.

2. Crackle Pattern: The glazed surface should have a natural, well-spaced crackle pattern (ice crackle). The deeper "iron wire" cracks intertwine with the finer, shallower "golden thread" cracks, infusing the static vessel with boundless vitality.

3. "Purple Mouth and Iron Foot": Due to the iron content in the clay body, the thinly glazed rim shows a purplish-brown tone, known as "purple mouth." The unglazed foot ring appears deep gray-brown or blackish-brown, the "iron foot." The "dark-colored foot ring" you mentioned is precisely this characteristic.

4. Spiral Pattern Craftsmanship: The spiral patterns on the neck should slightly protrude from the glazed surface, with smooth, rounded lines that can be felt by touch. They are not merely decoration but also enhance the visual height of the neck.

Conclusion: A Masterpiece of Potent Visual Tension

In summary, the spiral-patterned bottle you describe, through its extreme比例 (proportion) of 12 cm versus 3 cm, represents the pinnacle of Northern Song Guan ware's exploration in form. It carries the solemnity of ancient ritual vessels while embodying the simplicity,挺拔 (tall and straight), and philosophical contemplation revered in the Song period, making it a paradigm of the "unity of Dao and artifact."

If this object is before you, it is a masterpiece worthy of in-depth study and appreciation. For such pieces, it is usually necessary to examine微观特征 (micro-features) like glaze bubbles and clay body texture firsthand, combined with archaeological findings for comprehensive analysis.

If you can provide or observe more specific details, such as the形态 (form) of the crackle pattern, the specific shade of the glaze (leaning towards blue or green), or the number and shape of the spur marks on the base, we could further discuss its possibly more specific kiln origin or period characteristics.